Exhibit 4.1

Okeanis Eco Tankers Corp.

Trust Company Complex

Ajeltake Island

Majuro MH96960

Marshall Islands

17 November 2025

Strictly private and confidential

Dear Sirs,

Re: Engagement letter for investment banking services

We are pleased to confirm the terms under which we are engaged to work with you and provide investment banking services in relation to the matters set forth below.

The services which we are engaged to perform shall be referred to as the “Engagement” and this letter as the “Engagement Letter”.

The purpose of this Engagement Letter is to set forth the terms and conditions under which Fearnley Securities AS (“Fearnley”), as Global Coordinator, and Clarksons Securities AS (“Clarksons”), as Joint Bookrunner, jointly referred to as the “Managers”, are engaged by Okeanis Eco Tankers Corp. (the “Company”) to act as its financial advisors in connection with an intended raising of USD 100 million through the issuance of new shares (the “Transaction”) which may be completed in one or multiple steps, the purpose of which is to fund the equity portion of the Company’s acquisitions of additional tanker vessels and general corporate purposes.

This Engagement Letter is supplemented by Fearnley’s Standard Terms of Assignment (the “Standard Terms”) and Fearnley’s General Business Terms, both of which are available at Fearnley’s web site (www.fearnleysecurities.com) (and are incorporated herein and constitute a part hereof). Reference is also made to Fearnley’s “Pricing, Placing and Allocation Policy” for important disclosures in terms of transaction execution and the services to be provided as part of the Engagement, as also available on the web site.

To the extent that the Engagement shall include any sales to clients in the US, the Managers will involve its affiliate companies in the US that are registered broker dealers under applicable US law (which in the case of Fearnley is Fearnley Securities Inc. and in the case of Clarksons is Clarksons Securities Inc.). In such case, it shall be understood that such affiliate companies shall be covered by the terms of this Engagement Letter including any representations and warranties of the Companies thereunder, and that such affiliate companies shall be regarded as Managers for the purpose of the Engagement.

In the event of discrepancies between the Engagement Letter and the Standard Terms or the General Business Terms, the Engagement Letter shall prevail. The Standard Terms shall apply mutatis mutandis to each of the Managers and any reference to Fearnley shall include each of the Managers.

The Company makes the representations and warranties as set forth in Annex I. Each of the parties makes the covenants and agreements set forth on Annex II.

The Managers shall, in accordance with what is considered standard market practice for their respective roles in the Engagement, provide the following services to the Company as part of the Engagement:

The Managers shall not be liable for the inability to successfully complete the Transaction, whether on grounds of market conditions or for any other reason.

Unless specifically agreed, the Managers shall not have an obligation to:

The Managers shall not have any obligation to provide tax, securities law or other legal, regulatory, accountancy or technical advice or services, and no information provided by the Managers may be considered as such. It is the responsibility of the Company obtain such advice as it may require.

The Company confirms that, to the extent required under applicable law, it has considered the equal treatment of its shareholders when deciding that the Transaction should be carried out as a registered direct offering as opposed to a rights offering to its existing shareholders. The Company furthermore confirms that its senior management has the necessary knowledge and experience to make this decision, or, if relevant, its senior management has explicitly and clearly made the Managers aware of their lack of knowledge and experience in this regard. The Managers assume no responsibility for the Company’s obligation with respect to its equal treatment of its shareholders.

The liability of the Managers under this Engagement Letter shall be several and not joint. The rights of each of the Managers under this Engagement Letter may be enforced separately by each of them. No compromise or waiver by a Manager will affect the rights of or otherwise bind the other Managers. No Manager has any authority to act on behalf of any other Manager unless expressly authorized to do so.

As remuneration for the services to be provided by the Managers under the Engagement, the Company agrees to pay the following fees, subject to successful completion of a Transaction:

Proceeds, whether in cash or in-kind from shares allocated to the Alafouzos family, and/or their direct affiliates, in the Transaction shall be excluded from the calculation of both the Transaction Fee and the Discretionary Fee.

The Managers shall have the right to withhold any amounts due to the Managers in relation to the Engagement (such as Transaction Fee, Discretionary Fee and applicable expenses) in connection with the release of net proceeds to the Company.

The fees shall be payable regardless of the number of tasks set out in section 2 which have actually been performed. If the Transaction is carried out in more than one step/tranche, each step/tranche shall entitle the Managers to fees as set out above.

The Company’s obligation to cover the Manager’s expenses are set forth in the Standard Terms and shall, for avoidance of doubt, include an obligation to cover, as applicable, the Manager’s expenses related to bookrunner software (Dealogic) and market sounding software (Logwise) at the prevailing cost. With respect to other third-party expenses, the Managers shall always seek the Company’s authorization before incurring any material costs or expenses hereunder. Notwithstanding the above, the costs related to the Transaction, excluding Managers’ counsel, shall be capped at USD 20,000, otherwise the Managers will seek approval by the Company, and a single expense related to the Transaction over USD 1,500 shall require approval by the Company.

As a condition to the several obligations of the Managers under the Engagement Letter and other transaction documents contemplated herein, the Company shall satisfy the conditions and comply with the covenants set forth in Annex II unless waived by both of the Managers.

The Engagement shall terminate upon the earlier of (i) the conclusion of the Transaction and (ii) after 4 months following the signing of this Engagement Letter.

The Engagement Letter and the Engagement may be terminated with or without cause by the Company or by the Managers by written notice at any time.

The following provisions shall survive any termination of the Engagement Letter and remain in full force and effect:

In addition, where (i) the Company terminates the Engagement without cause or (ii) the Managers terminate the Engagement with cause, the Managers’ right to fees pursuant to this Engagement Letter shall survive and remain in full force and effect and become payable where, within 6 months from termination, any transaction to which this Engagement Letter would have applied is resolved, agreed (whether subject to conditions or otherwise) or concluded by the Company. Such right to fees will not apply in the event the Managers have been offered by the Company to assist with a transaction following termination on terms not less favourable than the terms of this Engagement, but the Managers have declined to participate in.

The provision of investment banking services or ancillary services by the Managers entails an obligation for each Manager to have the Company registered as a client, including the obligations under the Act of 1 June 2018 no. 23 regarding measures to combat laundering and terrorist financing (the Anti-Money Laundering Act) and appurtenant regulations. Accordingly, this Engagement Letter shall not be binding upon the Managers until and unless the completion of such satisfactory client registration. As part of such client registration, the Managers will perform an evaluation of the Company’s degree of sophistication (non-professional, professional or eligible counterparty) which will have impact on the level of client protection that the Managers will be required to provide under EU/EEA legislation. The Company is responsible for keeping the Managers informed about any changes that could affect its classification.

Fearnley’s provision of investment banking services is subject to its General Business Terms and Conditions which are available on its web site. General terms and conditions as well as additional information with respect to investment banking services provided by Clarksons are available on Clarksons’ web site.

The Company notes the application of the requirements of Article 9(8) of the Product Governance Rules under EU Delegated Directive 2017/593 (the “MiFID II Product Governance Rules”) and acknowledges that the target market and distribution channels will be identified by the Managers for the Transaction.

The Company notes the application of the sustainability requirements under the Regulation (EU) 2019/2088 (SFDR) and acknowledges the scope and content of sustainability-related information that must be communicated and presented to investors.

For the purposes of the requirements of the MiFID Product Governance Rules regarding the mutual responsibilities of manufacturers under the MiFID Product Governance Rules, each of the Managers acknowledges to each other that it understands the responsibilities conferred upon it under the MiFID Product Governance Rules relating to each of the product approval process, the target market and the proposed distribution channels for the Transaction.

This Engagement Letter shall be governed and construed in accordance with Norwegian law.

Any dispute arising in connection with this Engagement Letter shall be settled by the ordinary courts with the Oslo District Court as the (non-exclusive) legal venue. Clients with a foreign legal venue waive any right they have to oppose a lawsuit that is related to this Engagement Letter being heard by the Oslo District Court. Clients with a legal venue abroad may, irrespective of the above, be sued by the Manager in such a legal venue should the Manager wish to do so.

The Company represents and warrants that the execution of this Engagement Letter has been approved and authorized by its relevant corporate bodies, and the persons signing the Engagement Letter on behalf of the Company confirm that they have the express power and authority to engage the Managers on the terms hereof.

For the avoidance of doubt, the Company agrees that the Engagement shall not restrict the Managers from engaging in similar transactions for other clients or to advise prospective investors in a transaction in relation to other investments. Fearnley’s policy for conflicts of interest is available on its web site (www.fearnleysecurities.com).

*****

Please confirm your agreement to the terms of the Engagement by signing this Engagement Letter and returning it to us.

Yours faithfully,

For Fearnley Securities AS

For Clarksons Securities AS

Agreed and accepted

For Okeanis Eco Tankers Corp.

Annex I

Representations and Warranties of the Company

In connection with the proposed sale to certain investors (the “Direct Offering”) of shares of common stock, par value $0.001 per share (the “Shares”), of the Company, the Company is providing the Managers with certain representations and warranties set forth in this Annex I.

The Company has filed with the Securities and Exchange Commission (the “Commission”) a registration statement, including a prospectus (File No. 333-287032) on Form F-3 relating to the securities (the “Shelf Securities”), including the Shares, to be issued from time to time by the Company. The registration statement as amended to the date of the Engagement Letter, including the information (if any) deemed to be part of the registration statement at the time of effectiveness pursuant to Rule 430A or Rule 430B under the Securities Act of 1933, as amended (the “Securities Act”), is hereinafter referred to as the “Registration Statement”, and the related prospectus covering the Shelf Securities dated May 21, 2025 in the form first used to confirm sales of the Shares (or in the form first made available to the Managers by the Company to meet requests of purchasers pursuant to Rule 173 under the Securities Act) is hereinafter referred to as the “Basic Prospectus.” The Basic Prospectus, as supplemented by the prospectus supplement specifically relating to the Shares in the form first used to confirm sales of the Shares (or in the form first made available to the Managers by the Company to meet requests of purchasers pursuant to Rule 173 under the Securities Act) is hereinafter referred to as the “Prospectus,” and the term “preliminary prospectus” means any preliminary form of the Prospectus. For purposes of this Annex I, “free writing prospectus” has the meaning set forth in Rule 405 under the Securities Act, “Time of Sale Prospectus” means the documents and pricing information identified in Schedule I hereto, and “broadly available road show” means a “bona fide electronic road show” as defined in Rule 433(h)(5) under the Securities Act that has been made available without restriction to any person. As used herein, the terms “Registration Statement,” “Basic Prospectus,” “preliminary prospectus,” “Time of Sale Prospectus” and “Prospectus” shall include the documents, if any, incorporated by reference therein as of the date hereof. The terms “supplement,” “amendment,” and “amend” as used herein with respect to the Registration Statement, the Basic Prospectus, the Time of Sale Prospectus, any preliminary prospectus or the Prospectus shall include all documents subsequently filed by the Company with the Commission pursuant to the Securities Exchange Act of 1934, as amended (the “Exchange Act”), that are deemed to be incorporated by reference therein pursuant to Item 6 of Form F-3.

The Company represents and warrants to and agrees with each of the Managers that:

Annex II

Conditions to the Managers’ Obligations and Covenants of the Company and the Managers

1. As a condition to the several obligations of the Managers under the Engagement Letter and other transaction documents contemplated herein, the Company shall meet the following conditions unless waived by both of the Managers:

2. The Company covenants with each Manager as follows:

Each Manager agrees, severally and not jointly, to indemnify and hold harmless the Company, the directors of the Company, the officers of the Company who sign the Registration Statement and each person, if any, who controls the Company within the meaning of either Section 15 of the Securities Act or Section 20 of the Exchange Act from and against any and all losses, claims, damages and liabilities (including, without limitation, any legal or other expenses reasonably incurred in connection with (x) defending or investigating any such action or claim) caused by any untrue statement or alleged untrue statement of a material fact contained in the Registration Statement or any amendment thereof, any preliminary prospectus, the Time of Sale Prospectus or any amendment or supplement thereto, any issuer free writing prospectus as defined in Rule 433(h) under the Securities Act, any Company information that the Company has filed, or is required to file, pursuant to Rule 433(d) under the Securities Act or the Prospectus or any amendment supplement thereto, or caused by any omission or alleged omission to state therein a material fact required to be stated therein or necessary to make the statements therein not misleading, but only with reference to information relating to such Manager furnished to the Company in writing by such Manager through you expressly for use in the Registration Statement, any preliminary prospectus, the Time of Sale Prospectus, any issuer free writing prospectus, or the Prospectus or any amendment or supplement thereto and (y) any Manager violating or causing the Company to violate, the Securities Act, including by not appropriately involving a registered broker dealer under applicable US law with respect the offering described in this Engagement Letter.

Schedule I

Schedule II

Exhibit 4.2 

General Information: Okeanis Eco Tankers Corp., a corporation incorporated under the laws of the Republic of Marshall Islands (registration number 96382, LEI code 213800U35RCYXTKVEM65) (the “Company” or the “Issuer”), with shares that are primarily listed on the New York Stock Exchange under the trading symbol “ECO” and registered under SEDOL BLR8SN2, and secondary listed on Euronext Oslo Børs under the trading symbol “OET” and registered under ISIN MHY641771016, intends to offer new common shares in the Company, each with a par value of USD 0.001 (the “Offer Shares”) to raise gross proceeds of approximately USD 100 million (the “Offering”).

Offer price: The offering price per Offer Share will be determined in USD on the basis of an accelerated bookbuilding process (the “Offer Price”). The final number of Offer Shares to be issued will be determined by the Company’s board of directors or a committee thereof (the “Board”) on the basis of the bookbuilding process, in consultation with the Managers (as defined below).

The Managers: The Company has appointed Fearnley Securities AS as global coordinator and joint bookrunner, and Clarksons Securities AS as joint bookrunner (collectively referred to as the “Managers”) in the Offering.

Use of Proceeds: The Company intends to use the net proceeds from the Offer Shares to partly finance the acquisition of two Suezmax tanker newbuilding resales, both expected for delivery during January 2026, to be acquired from unrelated third parties for an agreed price of USD 97 million per vessel when delivered (the “Vessel Acquisitions”), subject to, inter alia, successful completion of the Offering. If one or both of the Vessel Acquisitions does not consummate, the net proceeds from the Offering may be used for general corporate purposes.

Documentation: The principal terms and conditions of the Offering are set out in the term sheet (the “Term Sheet”), which is circulated together with this application agreement (including Exhibit I) (the “Application Agreement”), and in a preliminary prospectus supplement dated 18 November 2025 (and the documents incorporated by reference therein (the “Prospectus Supplement”)). The Term Sheet and the Prospectus Supplement shall together constitute the “Investor Documentation”. The applicant (the “Applicant”) hereby acknowledges to have received and accepted the terms set out in the Investor Documentation and that the application and subscription is subject to the terms set out therein and in this Application Agreement.

Investors are deemed to have access to information about the Company that is publicly available, including financial information and other relevant information about the Company, stock exchange announcements, periodic reports (including the Company’s latest interim report for the nine months period ended 30 September 2025) and other filings to the date hereof, as available on the Company’s web site or made available through the U.S. Securities and Exchange Commission’s (the “SEC”) website at www.sec.gov and the Oslo Stock Exchange’s information system www.newsweb.no. Information on or accessed through these websites does not constitute a part of the Investor Documentation and, unless specifically stated otherwise, is not incorporated by reference therein or herein.

The Prospectus Supplement has been filed with the SEC under its shelf registration statement on Form F-3 (File No. 333-287032) in connection with the Offering and may be obtained at www.sec.gov. The Company plans to file a final prospectus supplement with the SEC under Rule 424(b) under the U.S. Securities Act of 1933 in connection with the Offering (the “Final Prospectus Supplement”).

Limitation of liability: The Managers disclaim any liability, to the fullest extent permitted, for the accuracy or completeness of the information in the Investor Documentation. Further, the Managers disclaim any liability for all other information (whether written or oral) concerning the Company, the Offer Shares or the Offering, irrespective of whether such information was received through the Managers, the Company or otherwise, all to the extent legally permissible. Notwithstanding the above, if the Applicant has received information from the Company or the Managers for the purposes of conducting its own due diligence investigations, the Applicant accepts that all information set out in the Investor Documentation is, to the fullest extent permitted by law, provided on a strictly non-reliance basis. By signing this Application Agreement or by making an application for the Offer Shares on the basis of this Application Agreement, the Applicant warrants that it understands and accepts that it is applying for the Offer Shares and participating in the Offering on these terms and conditions and that the Applicant has not been induced to enter into this Application Agreement by any representation, warranty or undertaking by any of the aforementioned.

Conditionality of the Offering: The completion of the Offering by delivery of Offer Shares to Applicants is subject to (i) all necessary corporate resolutions of the Company required to implement the Offering being validly made, including without limitation, the Board approving the allocation and issuance of Offer Shares, (ii) the issuance of relevant documents and legal opinions for the issuance of the Offer Shares as required in the engagement letter entered into by the Managers and the Company, unless waived by the Managers, and (iii) the agreements for the Vessel Acquisitions remaining in full force and effect at the time of settlement of the Offer Shares. The completion of the Offering is not conditional upon the completion of the Vessel Acquisitions. Items (i) to (iii) in the foregoing are referred to as the “Conditions”. Up until notice of allocation of Offer Shares in accordance with this Application Agreement, the Offering may be cancelled by the Company in its sole discretion for any reason. Neither the Managers nor the Company will be liable for any losses if the Offering is cancelled, irrespective of the reason for such cancellation.

Minimum subscription and allocation: The minimum subscription and allocation amount in the Offering will be a number of Offer Shares corresponding to the USD equivalent of EUR 100,000. The Company may, at its sole discretion, allocate Offer Shares for an amount below EUR 100,000 to the extent exemptions from relevant prospectus requirement pursuant to Regulation (EU) 2017/1129 on prospectuses for securities (the “EU Prospectus Regulation”) as well as the UK Prospectus Regulation (as defined in the below), are available.

Application procedure: Applications will take place from and including 18 November 2025 at 22:00 hours CET to and including 19 November 2025 at 11:00 hours CET (the “Application Period”). The Company reserves the right, at its own discretion, to close or extend the Application Period at any time and for any reasons and on short notice. If the Application Period is shortened or extended, the other dates referred to herein may be amended accordingly.

To facilitate an efficient bookbuilding process, a trading halt will be imposed in the Company’s shares that are trading on Euronext Oslo Børs throughout the bookbuilding period and until final results have been announced, expected no later than 13:00 CET on 19 November 2025.

By executing this Application Agreement, or by placing an application by taped phone, email, the messenger service of Bloomberg or any other electronic communication as further described below, the Applicant irrevocably confirms the Applicant’s request to subscribe for the number of Offer Shares at the amount(s) specified by such Applicant on the terms included in the Investor Documentation, and authorizes and instructs the Managers or its appointed representative, each acting alone, to subscribe for the number of Offer Shares allocated to the Applicant in the Offering (the “Allocated Shares”) on behalf of the Applicant. If no price limit is stated, no price limit will apply and the application will be considered as an application for the stated number of Offer Shares or the stated amount at any final Offer Price.

This Application Agreement, duly signed, valid and binding on the Applicant, must be received by one of the Managers by the end of the Application Period. The Applicant bears the risk of any delays, unavailable digital systems and channels and any other technical problems. The Applicant is furthermore responsible for the correctness of the information provided by the Applicant in this Application Agreement. However, the Managers may, in their sole discretion, accept applications placed by taped phone, e-mail, on Bloomberg or otherwise within the Application Period (but may request that the application is subsequently confirmed by the execution of an Application Agreement in writing, and may, if the Applicant fails to satisfy such requirement, in their sole discretion, disregard the application, without any liability towards the Applicant). Any application received by a Manager (whether in writing (through email, Bloomberg or otherwise) or by taped phone) becomes binding at the end of the Application Period and may not be withdrawn or amended after such time.

ANY APPLICATION PLACED BY TAPED PHONE, E-MAIL, ON BLOOMBERG OR OTHERWISE WILL BE DEEMED MADE ON THE TERMS AND SUBJECT TO THE CONDITIONS SET OUT IN THIS APPLICATION AGREEMENT (INCLUDING, FOR THE AVOIDANCE OF DOUBT, EXHIBIT I).

Allocation of Offer Shares: Notification of allocation and payment instruction (the “Notification”) will be sent to the Applicant by the Managers on or about 19 November 2025, subject to any shortening or extensions of the Application Period. The allocation of Offer Shares will be made at the discretion of the Board in consultation with the Managers. The Board will focus on criteria such as (but not limited to) existing ownership, pre-commitments and indications from the pre-sounding phase of the Offering, timeliness of the application, price leadership, relative order size, sector knowledge, perceived investor quality and investment horizon. The Board, in consultation with the Managers, further reserve the right, at their sole discretion, to take into account the creditworthiness of any applicant. The Company may, at its sole discretion, set a maximum allocation to any Applicant as well as reject or reduce any application in whole or in part. There is no guarantee that any Applicant will be allocated Offer Shares. Allocation of Offer Shares totalling a lower amount than applied for does not affect the Applicant’s obligation to subscribe and pay for the Offer Shares allotted.

Settlement: The date for settlement of the Offering is expected to be on or about 21 November 2025 (the “Settlement Date”), subject to any shortening or extensions of the Application Period, and any further settlement details will be stated in the Notification. The Applicant shall pay the subscription amount (being the number of Allocated Shares multiplied with the Offer Price) in accordance with the procedures set out herein and in the Notification. The Managers assumes no responsibility for the delivery and payment obligations of the Company and Applicant respectively. The Allocated Shares will be delivered to the Applicant through The Depository Trust Company (the “DTC”) on a delivery versus payment (DVP, T+2) basis as soon as practicable after the Conditions have been met. The Offer Shares will be available for trading on the New York Stock Exchange once the Offer Shares have been issued and delivered in the DTC and settlement has taken place. To the extent the subscription amount for the Allocated Shares is payable or paid by the Applicant to an interim account of the Managers, the Applicant irrevocably authorizes and instructs the Managers or its appointed representative, each acting alone, to release such amount to the Company.

DTC account: Any allocation of Offer Shares is conditional upon the Applicant holding a DTC brokerage account. The DTC brokerage account details must be stated in this Application Agreement. Applicants who are unable, or do not wish, to take delivery of Offer Shares in the DTC should contact the Managers to discuss alternative arrangements for delivery of Offer Shares in Euronext Securities Oslo (VPS).

Confidentiality: The offer to subscribe for Offer Shares in the Offering is personal and cannot be forwarded or made known to any third party. The Applicant hereby undertakes to keep the contents of this Application Agreement and any information made available pursuant to it confidential, including, but not limited to, the fact that any agreement has been entered into until the completion of the Offering has been resolved and publicly announced by the Company, with the exemption for disclosure to applicable authorities as required by law. The Applicant hereby authorises the Managers to produce this Application Agreement or a copy hereof to any party in any administrative or legal proceedings or official inquiry with respect to matters covered hereby in connection with the Offering, to the extent required by law. The Applicant further acknowledges that a form of this Application Agreement will be filed with the SEC, subject to completion of the Offering.

Confirmations: The Applicant, by applying for Offer Shares and thereby accepting the terms of this Application Agreement (including Exhibit I) confirms its request to purchase and pay for the allocated and subscribed number of Offer Shares and further confirms that:

[SPECIFICATION OF APPLICATION FOLLOWS ON THE NEXT PAGE]

SPECIFICATION OF APPLICATION

Please specify the number of Offer Shares or USD amount applied for, subject to the price limitations set forth below. If no specification is given below, the “no price limit” option will apply. Please state clearly whether the Application is in number of Offer Shares or amount.

INFORMATION ON THE APPLICANT – ALL FIELDS MUST BE COMPLETED

In order to deliver the Offer Shares to the Applicant’s DTC brokerage account or the DTC brokerage account of a designee, the below fields must be correctly completed. For the avoidance of doubt, a designee must qualify as an “Affiliate”1 or “Related Fund”2 of the Applicant in order to receive the Offer Shares in the DTC.

The Company and the Managers have the right to disregard the application, without any liability towards the Applicant, if any compulsory information requested in this Application Agreement is not provided. Notwithstanding the aforementioned, any compulsory information is not filled in by the Applicant, the Company and the Managers reserve the right to obtain such information through publicly available sources and use such information in this Application Agreement.

Please note: If this Application Agreement is sent to the Managers by e-mail, the e-mail will be unsecured unless the Applicant itself takes measures to secure it. This Application Agreement may contain sensitive information, and the Managers recommend the Applicant to send this Application Agreement to the Managers in a secured e-mail. Please refer to Exhibit I for further information on the Managers’ processing of personal data.

The Applicant hereby acknowledges to have received and accepted the terms set out in this Application Agreement (including Exhibit I) and that the application and subscription is subject to the terms set out herein.

EXHIBIT I

Terms and conditions of application

Selling and transfer restrictions:

General: This Application Agreement does not constitute an offer to sell or a solicitation of an offer to buy Offer Shares in any jurisdiction in which such offer or solicitation is unlawful or where this would require registration, publication of a prospectus or similar action.

No EEA or UK prospectus: The Investor Documentation or any other material related to the Offer Shares does not constitute or form part of a prospectus within the meaning of the EU Prospectus Regulation, as implemented in any member state of the European Economic Area (the “EEA”) (each, a “Relevant Member State”) and the United Kingdom. The expression “EU Prospectus Regulation” means in relation to the EU/EEA Regulation (EU) 2017/1129 (and amendments thereto) and in relation to the United Kingdom the Regulation (EU) 2017/1129 as it forms part of the United Kingdom (“UK”) domestic law by virtue of the European Union Withdrawal Act 2018 (the “UK Prospectus Regulation”) and includes any relevant implementing measure in each Relevant Member State. The Investor Documentation or any other material related to the Offer Shares has therefore not been, and will not be, reviewed by or registered with the Financial Supervisory Authority of Norway (No. Finanstilsynet) or any other regulator or public authority. Accordingly, the Offer Shares will only be offered or sold within the EEA in reliance on applicable exemptions from preparing a prospectus pursuant to the EU Prospectus Regulation and in the United Kingdom according to applicable exemptions under the UK Prospectus Regulation, together with any connected legislation for member states of the EEA or the UK, as applicable.

United Kingdom: Each UK Applicant confirms that it understands that the Offering has only been communicated (a) to persons who have professional experience, knowledge and expertise in matters relating to investments and are “investment professionals” for the purposes of article 19(5) of the Financial Services and Markets Act 2000 (Financial Promotion) Order 2005 (all such persons being referred to as “relevant persons”) and (b) only in circumstances where, in accordance with section 86(1)(c) and (d) of the Financial and Services Markets Act 2000 (“FSMA”), the requirement to provide an approved prospectus in accordance with the requirement under section 85 FSMA does not apply as the minimum denomination of and purchase of the Offer Shares exceeds EUR 100,000 or an equivalent amount. Consequently, the Applicant understands that the Offer Shares may be offered only to “qualified investors” for the purposes of sections 86(1) and 86(7) FSMA, or to limited numbers of UK investors, or only where minima are placed on the consideration or denomination of securities that can be made available. Any application or purchase of Offer Shares is available only to relevant persons and will be engaged in only with relevant persons and each UK Applicant warrants that it is a relevant person.

Canada: The distribution of the Offer Shares in Canada is being made only on a private-placement basis, thus exempting it from the requirement that the Company prepare and file a prospectus with the applicable securities regulatory authorities. The Offer Shares are being offered in those jurisdictions and to those persons where and to whom they may lawfully be offered for sale, and therein only by persons permitted to sell such securities. Each Canadian purchaser who purchases Offer Shares must be entitled under applicable securities laws to purchase such securities without the benefit of a prospectus qualified under such securities laws; must be an “accredited investor” within the meaning of National Instrument 45-106 – Prospectus and Registration Exemptions and purchasing the Offer Shares as principal or deemed principal for its own account; and must be a “permitted client” within the meaning of National Instrument 31-103 – Registration Requirements and Exemptions. There is currently no public market for the Offer Shares in Canada and any resale of the Offer Shares in Canada must be made in accordance with applicable securities laws.

Australia and Japan: The Offer Shares will not be registered under the applicable securities laws of Australia or Japan and may not be offered, sold, resold or delivered, directly or indirectly, in or into Australia or Japan except pursuant to an applicable exemption from applicable securities laws.

Switzerland: The Offering is not intended to constitute, and does not constitute, an offer to the public or solicitation to purchase or invest in the Offer Shares. The Offer Shares may not be publicly offered, sold or marketed, directly or indirectly, in or into Switzerland within the meaning of the Swiss Financial Services Act (“FinSA”), except under the following exemptions under the FinSA: (i) to any investor that qualifies as a professional client within the meaning of the FinSA; (ii) in any other circumstances falling within Article 36 FinSA, provided, in each case, that no such offer of Offer Shares referred to in (i) and (ii) shall require the publication of a prospectus for offers of Offer Shares pursuant to the FinSA. The Offer Shares have not been and will not be admitted to trading on any trading venue in Switzerland. Neither the Investor Documentation nor any other marketing or offering material relating to the Offer Shares constitutes a prospectus within the meaning of the FinSA, and has not been, and will not be, filed with, or reviewed or approved by, a Swiss review authority, and does not comply with the disclosure requirements applicable to a prospectus within the meaning of the FinSA. Neither this Investor Documentation nor any other offering or marketing material relating to the Offer Shares may be distributed or otherwise made available in Switzerland in a manner which would require the publication of a prospectus in Switzerland pursuant to the FinSA.

Regulatory issues: The Managers are required to categorize all new clients in one of three categories: Eligible counterparties, professional, and retail clients. All Applicants who are not existing clients of a Manager will be categorized as retail unless otherwise is communicated in writing by the relevant Manager. For further information, the Applicant may contact the relevant Manager. The Managers will treat the Application as an execution only instruction from the Applicant to apply for Offer Shares under the offer and hence the Managers will not determine whether the Application for Offer Shares is suitable or not for the Applicant.

Personal data: The Applicant’s personal data will be processed confidentially and according to legal obligations. Personal data will only be shared as far as necessary to fulfil this agreement/transaction. Supplementary information on processing of personal data and the Applicants’ rights can be found on the Managers’ websites.

Managers’ consideration: The Managers will, subject to completion of the Offering, receive consideration from the Company for carrying out their assignment as Managers in line with market practice.

Legal Entity Identifier (“LEI”) and National Client Identifier (“NID”): Applicants that are legal entities are required to submit a LEI. A LEI is a 20-digit, alpha-numeric code that enables clear and unique identification of legal entities participating in financial transactions. LEIs, like other identifiers, are needed by the Managers to fulfil certain reporting obligations under financial regulations and directives. LEIs are also key for matching and aggregating market data, both for transparency and regulatory purposes. The code is linked to a set of key reference information relating to the legal entity in question e.g., name and address. Once a legal entity obtains a LEI code, the code is assigned to that legal entity for its entire life. A LEI number may be obtained by contacting the preferred LEI issuing organisation (LEI issuer, also known as Local Operating Unit). The list of LEI issuers is available on the Global LEI Foundation (GLEIF) website https://www.gleif.org/en/.

Applicants that are natural persons are required to submit their NID. The appropriate form of NID will depend on the home country of the Applicant. An exhaustive list of countries and corresponding form of NID is set out in Annex 2 of Commission Delegated Regulation 2017/590.

Information exchange and barriers: The Applicant acknowledges that there is a duty of secrecy between the different units of a Manager as well as between a Manager and the other entities in such Manager’s group. This may entail that other employees of a Manager or a Manager’s group may have information that may be relevant to the Applicant, but which a Manager will not have access to in its capacity as Manager for the Offering. The Managers are part of securities firms that offer a broad range of investment services. In order to ensure that assignments undertaken in certain departments are kept confidential, the other activities, including analysis and stock broking, are separated from the respective departments by information walls. The Applicant acknowledges that the analysis and stock broking activity within the securities firms may conflict with the Applicant’s interests with regard to transactions in the Offer Shares as a consequence of such information walls.

Mandatory anti-money laundering procedures: The Offering is subject to applicable anti-money laundering legislation, including the Norwegian Money Laundering Act of 1 June 2018 no. 23 and the Norwegian Money Laundering Regulation of 14 September 2018 no. 1324 (collectively the “Anti-Money Laundering Legislation”). Applicants who are not currently registered as customers of a Manager may, if applicable, be subject to customer due diligence measures (“KYC”) to comply with Anti-Money Laundering Legislation. Applicants who have not completed the required KYC (if any) prior to the expiry of the Application Period may not be allocated Offer Shares.

Commission: It is not allowed to apply or subscribe for Offer Shares by commission or similar arrangements.

Cancellation: The Applicant acknowledges that the Offering will be cancelled if the Conditions are not fulfilled and may be cancelled and/or modified by the Company in its sole discretion for any other reason. The Managers will not be liable for any losses if the Offering is cancelled, irrespective of the reason for such cancellation.

Relation to law, regulations and by-laws: The Applicant has full power and authority to execute and deliver this Application Agreement and to approve these terms and conditions and to apply and subscribe for the Offer Shares and is authorised to pay all amounts it has committed to pay subject to the satisfaction of the terms stated herein for completion of the Offering. The execution and delivery of this Application Agreement has been authorised by all necessary action by the Applicant or on the Applicant’s behalf, and this Application Agreement shall constitute valid and binding obligations, enforceable against the Applicant in accordance with its terms. The Applicant bears the full risk for its legal ability to apply for, purchase for and own shares in the Company, and its monetary liability under this undertaking will not cease to be effective in the event that subscription and ownership of the Offer Shares would be illegal due to applicable statutory law and regulations. In such event, the Applicant shall fulfil the payment obligations that have been effected and will designate a third party to whom the Offer Shares are to be issued.

Subscriptions: The Managers reserve the right to apply for Offer Shares for an amount up to its respective agreed fees payable to the Managers by the Company in connection with the Offering. In the event a Manager applies for Offer Shares, the Company will, in consultation with such Manager, reduce allocations to such Manager in case of over-subscription in the Offering.

Overdue and missing payments: Overdue payments will be charged with interest at the applicable rate under the Norwegian Act on Interest on Overdue Payment of 17 December 1976 No. 100. A defaulting Applicant will be solely responsible for any deficit amount. The Managers reserve the right to advance payment on behalf of Applicants who have not paid for the Offer Shares allocated to the Applicant. A non-paying Applicant will remain fully liable for the subscription amount payable for the Offer Shares allocated to it, irrespective of any payment made on its behalf by a Manager. However, the Managers reserve the right to sell or assume ownership of Offer Shares without further notice to the Applicant in question if payment has not been received. If the Offer Shares are sold on behalf of the Applicant, the Applicant will be liable for any loss, costs, charges and expenses suffered or incurred by a Manager or the Company as a result of or in connection with such Offer Shares.

Target Market: The manufacturer Target Market (MIFID II product governance) for the Offering is non-professional, professional and eligible counterparties (all distribution channels, subject to the distributor’s suitability and appropriateness obligations under MiFID II, as applicable), who; b) have at least a common/normal understanding of the capital markets, c) are able to bear the losses of their invested amount and, d) are willing to accept risks connected with the Offer Shares, and e) have an investment horizon which takes into consideration the liquidity of the shares. The issuer for the Offering has not published sufficient data for the manufacturer to determine whether an investment in the Offering is compatible for investors who have expressed sustainability related objectives with their investments based on that which i) is an environmentally sustainable investment under the EU Taxonomy Regulation, ii) represents a sustainable investment under the SFDR, and/or iii) takes into consideration any Principle Adverse Impacts on sustainably factors as per the SFDR. The negative target market for the Offer Shares are investors that seek full capital protection or full repayment of the amount invested, are fully risk averse/have no risk tolerance or need a fully guaranteed income or fully predictable return profile.

Notwithstanding, and without affecting the manufacturers target market assessment as per the above, the Managers will only allow distribution through their distribution channels to investors who in the EU meet the requirements set out in the manufacturers target market assessment.

For distribution to investors located outside of the EU, distribution of the shares is only allowed to such investors which a) the Managers can approach as per the rules of the jurisdiction in which the investor reside, and b) which can provide adequate confirmations to this effect, and c) which as per minimum meets the requirements of the manufacturers target market assessment.

Third party rights: The terms and obligations in this Application Agreement are undertaken in favour of both the Company and the Managers in so far as is stipulated herein.

Governing law: The Offering and all related Investor Documentation shall be governed by Norwegian law, and any disputes (whether contractual or non-contractual) which cannot be resolved amicably, shall be referred to the ordinary courts of Norway and the Applicant accepts the non-exclusive jurisdiction of the Oslo District Court (No. Oslo tingrett).

United States: This Offering will be made only by means of the Application Agreement, the Term Sheet and the Prospectus Supplement and the accompanying base prospectus. A Prospectus Supplement related to the Offering has been filed with the SEC and is available on the SEC’s website located at www.sec.gov. Copies of the Prospectus Supplement and the accompanying base prospectus may be obtained from contacting Fearnley Securities AS at prospectus@fearnleys.com or Clarksons Securities AS at compliance.oslo@clarksons.com. This Offering will be made pursuant to the Company’s existing shelf registration statement on Form F-3 (Registration No. 333-287032) previously filed with the SEC and declared effective. This Application Agreement shall not constitute an offer to sell or the solicitation of an offer to buy these securities, nor shall there be any sale of these securities, in any state or other jurisdiction in which such offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of any such state or other jurisdiction. The Company plans to file the Final Prospectus Supplement with the SEC under Rule 424(b) under the U.S. Securities Act of 1933 in connection with the completion of the Offering.

Exhibit 5.1

November 20, 2025

Okeanis Eco Tankers Corp.: Exhibit 5.1 Opinion

Dear Sirs:

We have acted as counsel as to matters of the law of the Republic of the Marshall Islands (“Marshall Islands Law”) for Okeanis Eco Tankers Corp., a Marshall Islands corporation (the “Corporation”), in connection with the offering and sale by the Corporation of up to 3,239,436 shares of common stock, par value $0.001 per share (the “Common Shares”), being issued and sold by the Corporation in accordance with the Engagement Letter dated November 17, 2025, between the Corporation and the placement agents listed therein (the “Engagement Letter”), pursuant to the Corporation’s registration statement on Form F-3 (File No. 333-287032) (the “Registration Statement”), the base prospectus dated May 21, 2025 (the “Base Prospectus”), the preliminary prospectus supplement dated November 18, 2025 to the Base Prospectus (together with the Base Prospectus, the “Preliminary Prospectus”), and the final prospectus supplement dated November 19, 2025 to the Base Prospectus (together with the Base Prospectus, the “Final Prospectus”).

As such counsel, we have examined originals or copies (certified or otherwise identified to our satisfaction) of the following documents:

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In such examination, we have assumed (a) the legal capacity of each natural person, (b) the genuineness of all signatures and the authenticity of all documents submitted to us as originals, (c) the conformity to original documents of all documents submitted to us as conformed or photostatic copies, (d) that the documents reviewed by us in connection with the rendering of the opinion set forth herein are true, correct and complete and (e) the truthfulness of each statement as to all factual matters contained in any document or certificate encompassed within the due diligence review undertaken by us.

We have also assumed that:

This opinion is limited to the law of the Republic of the Marshall Islands. We expressly disclaim any responsibility to advise of any development or circumstance of any kind, including any change of law or fact that may occur after the date of this opinion letter that might affect the opinions expressed herein.

Based on the foregoing, and having regard to legal considerations which we deem relevant, and subject to the qualifications, limitations and assumptions set forth herein, we are of the opinion that the Common Shares, when issued, sold and paid for in accordance with the terms of the Registration Statement, the Preliminary Prospectus, the Final Prospectus, the Engagement Letter and the Application Agreement, will be validly issued, fully paid, and nonassessable.

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We hereby consent to the use of this opinion as an exhibit to a Report on Form 6-K of the Corporation, to the discussions of this opinion in the Registration Statement, the Preliminary Prospectus, and the Final Prospectus, and to the references to our firm in the Registration Statement, the Preliminary Prospectus, and the Final Prospectus. In giving such consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act of 1933, as amended, or the rules and regulations promulgated thereunder, nor do we admit that we are experts with respect to any part of the Registration Statement within the meaning of the term “expert” as used in the Securities Act.

Very truly yours,

Watson Farley & Williams LLP

/s/ Watson Farley & Williams LLP