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FORM 6-K

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

Report of Foreign Issuer

Pursuant to Rule 13a-16 or 15d-16 of

the Securities Exchange Act of 1934

For the month of October, 2017

Commission File Number: 001-12518

Banco Santander, S.A.

(Exact name of registrant as specified in its charter)

Ciudad Grupo Santander

28660 Boadilla del Monte (Madrid) Spain

(Address of principal executive office)

Indicate by check mark whether the registrant files or will file annual reports under cover of Form 20-F or Form 40-F:

Form 20-F  ☒            Form 40-F  ☐

Indicate by check mark if the registrant is submitting the Form 6-K in paper as permitted by Regulation S-T Rule 101(b)(1):

Yes  ☐            No  ☒

Indicate by check mark if the registrant is submitting the Form 6-K in paper as permitted by Regulation S-T Rule 101(b)(7):

Yes  ☐            No  ☒

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Banco Santander, S.A.

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Item 1

MATERIAL FACT

Banco Santander, S.A. discloses information in connection with the flexible compensation scheme “Santander Dividendo Elección” (scrip dividend scheme) to be applied to the second 2017 interim dividend. An informative document is enclosed for purposes of article 26.1.e) of Royal Decree 1310/2005, of 4 November (implementing Directive 2003/71/EC on the prospectus to be published when securities are offered to the public or admitted to trading).

Boadilla del Monte (Madrid), October 16, 2017

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INFORMATIVE DOCUMENT

CAPITAL INCREASE CHARGED TO RESERVES

BANCO SANTANDER, S.A.

October 16, 2017

THIS DOCUMENT HAS BEEN PREPARED IN ACCORDANCE WITH ARTICLE 26.1.E) OF ROYAL DECREE 1310/2005.

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The ordinary general shareholders’ meeting of Banco Santander, S.A. (“Banco Santander”, “Santander” or the “Bank”) held on April 7, 2017 resolved, under item six of its agenda, to increase the share capital of Banco Santander, with full charge to reserves, in an amount to be determined in accordance with the terms and conditions set out in the resolution (the “Increase”), delegating the execution of the Increase to the board of directors, with authority to delegate in turn to the executive committee, pursuant to article 297.1.a) of the consolidated Spanish Capital Corporations Law, as passed by means of Royal Legislative Decree 1/2010, of 2 July (“Spanish Capital Corporations Law”).

The executive committee of the Bank, in the meeting held on October 16, 2017, has resolved to execute the Increase in accordance with the terms and conditions set out by the ordinary general shareholders’ meeting, and has fixed the overall market value of the Increase (Amount of the Alternative Option) in 640 million Euro, pursuant to that resolution of the general shareholders’ meeting.

In accordance with article 26.1.e) of Royal Decree 1310/2005, of 4 November (implementing Directive 2003/71/EC on the prospectus to be published when securities are offered to the public or admitted to trading), the preparation and publication of a prospectus related to the admission to listing of the shares issued as a consequence of the execution of the Increase will not be necessary “provided that a document is made available containing information on the number and nature of the shares and the reasons for and details of the offer”. This informative document provides such information. This document is available at the Bank’s website (www.santander.com) and at the CNMV’s website (www.cnmv.es).

The Increase serves as an instrument for the shareholder-compensation scheme named “Santander Dividendo Elección” which shall be applied to the second 2017 interim dividend in October / November 2017. This scheme allows shareholders to opt between receiving newly issued Santander shares or an amount in cash broadly equivalent to the second interim dividend of 2017. The “Santander Dividendo Elección” scheme is similar to other programs implemented in the past by other international banks and extended among some of the companies which form part of the IBEX-35. With it Santander’s shareholders will be able to adapt their compensation to their preferences and personal situation, also benefiting from a more favourable tax treatment in case they opt to receive new shares.

“Santander Dividendo Elección” scheme works as follows. Each shareholder will receive a bonus share right for every Santander share held. These rights will be listed on and may be traded on the Spanish Stock Exchanges during a 15 calendar day period. Following the end of this period, the rights will be automatically converted into new Santander shares. Each shareholder may opt for one of the following alternatives:

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Additionally, shareholders will be able to combine the above mentioned alternatives in view of their specific needs.

Shareholders who do not make an election will receive new shares unless they have previously given (in previous programs) permanent instructions as described in section 3.5 below.

Pursuant to the formulae set forth in section 2 of the general shareholders’ meeting resolution, the executive committee has set the following details of the Increase:

Such number of rights has been calculated as follows, considering that the number of outstanding shares of the Bank is 16,040,573,446, that the Amount of the Alternative Option is 640,000,000 Euros, as indicated above, and that the average market price of the Santander share in the term set out in the capital increase resolution (PreCot) is 5.664 Euros:

Num. rights = NTAcc / (Amount Alternative Option / PreCot) = 16,040,573,446 / (640,000,000 / 5.664) = 141.95907499710 = 142 rights (rounded up to the nearest whole number).

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NAN = NTAcc / Num. rights = 16,040,573,446 / 142 = 112,961,784 new shares (rounded down).

To ensure that the number of bonus share rights necessary to receive a bonus share, and the number of bonus shares to be issued, are whole numbers, Pereda Gestión, S.A., a subsidiary of Banco Santander, has waived 118 bonus share rights, corresponding to 118 Santander shares owned by her.

In view of the number of shares to be issued set out above, the maximum amount of the Increase is 56,480,892 Euros. The amount by which the Bank’s capital will actually be increased is dependant upon the number of shares finally issued.

The amount of the Increase will be charged against the share premium reserves, which amounted to 44,912 million Euros as of December 31, 2016.

The gross price at which Banco Santander commits to purchase rights is 0.04 Euros per right, calculated pursuant to the formula set forth in the Increase resolution as follows:

Price of the commitment to purchase the bonus share rights = PreCot / (Num. of rights + 1) = 5.664 / (142 + 1) = 0.0396083916 = 0.04 (rounded up or down to the nearest Euro thousandth and, in case of a half Euro thousandth, rounded up to the nearest Euro thousandth).

As a result, those shareholders willing to receive their compensation in cash, will be able to sell their bonus share rights to Banco Santander at a fixed gross price of 0.04 Euros.

The envisaged calendar for the execution of the Increase is the following:

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The bonus share rights will be allotted to the shareholders of Banco Santander who have acquired their Santander shares no later than at 23:59h on the day of publication of the announcement of the Increase in the Official Bulletin of the Commercial Registry (“BORME”) (envisaged for October 17, 2017) and who appear as shareholders in the registries of Iberclear as at 23:59h on 19 October 2017. The trading period of the rights will begin on the next business day and will have a term of fifteen calendar days (from October 18 to November 1, 2017).

During the trading period of the rights, the shareholders may opt for cash or new shares as explained above, as well as for acquiring on market bonus share rights to subscribe for new shares. However, those shareholders who wish to accept the commitment to purchase rights offered by Banco Santander and receive cash at the guaranteed price shall need to communicate their decision no later than October 27, 2017. The commitment to purchase rights is addressed only at the rights allotted free of charge to shareholders, and not to those acquired on market. To choose among the alternatives offered by the “Santander Dividendo Elección” scheme, shareholders will have to contact the entities where their Santander shares and corresponding bonus share rights are deposited. Specifically:

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In any event, shareholders are urged to take into account the tax treatment of each of the options available to them, which are briefly summarized in section 5 below (and, in particular, the fact that withholding taxes may be applicable). The Increase is carried out free of fees and costs for shareholders in connection with the allotment of the new shares, with Banco Santander assuming the costs for the issue, subscription, placing on market, listing and other related costs.

The new shares to be issued in the Increase will be ordinary shares with a face value of fifty Euro cents (0.5) each, of the same class and series as those currently outstanding. The new shares will be issued at an issue price of fifty Euro cents (0.5), that is, without issuance premium, and will be represented in book-entry form, the records of which will be kept by Sociedad de Gestión de los Sistemas de Registro, Compensación y Liquidación de Valores, S.A.U. (Iberclear) and its participant entities.

The Increase is free of charge and, therefore, does not require any payment from the shareholders. As stated above, the Increase will be charged against the share premium reserves, which amounted to 44,912 million Euros as of December 31, 2016.

The balance sheet used for purposes of the Increase is that corresponding to December 31, 2016, duly audited by PricewaterhouseCoopers Auditores, S.L. on February 24, 2017 and approved by the ordinary general shareholders’ meeting on April 7, 2017 under item one A of its agenda.

Following the end of the trading period of the bonus share rights, the new shares that have not been capable of being allotted due to causes not attributable to Banco Santander will be kept in deposit and available to those who evidence lawful ownership of the relevant bonus share rights. Three years after the end of the bonus share rights trading period, the shares still pending to be allotted may be sold at the risk and expense of the interested parties in accordance with article 117 of the Spanish Capital Corporations Law. The net proceeds of the sale will be deposited in the Bank of Spain or in the General Deposit Bank (Caja General de Depósitos) at the disposal of the interested parties.

The new shares will confer the same voting and economic rights upon their holders as the currently outstanding ordinary shares of Banco Santander from the date on which the capital increase is declared to be subscribed and paid up, which is envisaged to happen on November 6, 2017.

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The Bank will apply for the listing of the new shares on the Madrid, Barcelona, Bilbao and Valencia Stock Exchanges through the Spanish Automated Quotation System (Mercado Continuo), and shall take the steps and actions that may be necessary with the competent bodies of the foreign Stock Exchanges on which Banco Santander shares are traded (currently Lisbon, London, Milan, Warsaw, Buenos Aires, Mexico, New York –through ADSs –American Depositary Shares– and São Paulo, through BDRs –Brazilian Depositary Receipts–) in order for the new shares issued under the Increase to be admitted to trading. Subject to the granting of the relevant authorizations, it is expected that the ordinary trading of the new shares in the Spanish Stock Exchanges will begin on November 15, 2017.

Below there is a brief description of the tax regime currently applicable in Spain to the options available for shareholders. This description does not constitute tax advice and does not include all tax considerations that may be relevant for each shareholder in view of his particular circumstances. Therefore, shareholders are advised to consult with their tax advisors the tax regime applicable to them.

The delivery of the shares issued in the Increase will be considered for tax purposes as a delivery of fully paid-up free-of-charge shares, and therefore, shall not be considered income for purposes of Personal Income Tax (Impuesto sobre la Renta de las Personas Físicas) (“IRPF”), Corporate Income Tax (Impuesto sobre Sociedades) (“IS”), or Non-Resident Income Tax (Impuesto sobre la Renta de no Residentes) (“IRNR”), whether or not the shareholders act through a permanent establishment in Spain.

The acquisition value, both of the new shares received in the Increase and of the shares from which they arise, will be the result of dividing the total cost by the applicable number of shares, both old and new. The acquisition date of the new shares will be that of the shares from which they arise.

If the shareholders sell their bonus share rights on the market, the amount so obtained will be taxed as follows in 2017:

In addition, for those transferors who are subject to IRPF and who are shareholders of Banco Santander, the amount obtained for the transfer of the bonus share rights will be subject to a withholding tax deduction (currently, 19%).

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In the event that the holders of the bonus share rights accept the Bank’s commitment to purchase the bonus share rights for a fixed price, the tax regime applicable to the amount obtained in the transfer to the Bank of the bonus share rights held in their capacity as shareholders will be that applicable to cash dividends and, therefore, shall be subject to applicable withholding and taxation.

It must be taken into account that the tax analysis above does not envisage all possible tax consequences of the various options available in connection with the “Santander Dividendo Elección” scheme. In particular, no reference is made to the tax consequences in their countries of residence for those shareholders who are not tax resident in Spain. Hence, shareholders are encouraged to consult their tax advisors with regard to the consequences of this remuneration scheme considering their particular situation, as well as to pay attention to potential changes of tax regulations and administrative constructions thereof that may take place.

Finally, holders of American Depository Receipts (ADRs), Brazilial Depository Receipts (BDRs) and Crest Depository Interests (CDIs) representing shares of Santander are encouraged to consult their tax advisors on the tax implications in Spain and abroad resulting from the specificities of these securities and from the various options available to them.

The options, terms and procedures indicated in this informative document may not be the same as those applicable to the shareholders owning Santander shares on the different foreign stock exchanges where the Bank is listed. These shareholders are urged to consult the public announcements made and other documents published in their jurisdictions.

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SIGNATURE

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.