Banco Santander S.A., trading as Santander Group (UK: /ˌsæntənˈdɛər, -tæn-/ SAN-tən-DAIR, -⁠tan-, US: /ˌsɑːntɑːnˈdɛər/ SAHN-tahn-DAIR, Spanish: [ˈbaŋko santanˈdeɾ]), is a Spanish multinational financial services company based in Santander, with operative offices in Boadilla del Monte. Additionally, Santander maintains a presence in most global financial centres as the 14th-largest banking institution in the world. Although known for its European banking operations, it has extended operations across North America and South America, and more recently in continental Asia. It is considered a systemically important bank by the Financial Stability Board.

Many subsidiaries, such as Abbey National, have been rebranded under the Santander name. The company is a component of the Euro Stoxx 50 stock market index. In June 2023, Santander was ranked as 49th in the Forbes Global 2000 list of the world's biggest public companies.

Banco Santander is chaired by Ana Patricia Botín-Sanz de Sautuola O'Shea, daughter and granddaughter of former chairmen Emilio Botin-Sanz de Sautuola y García de los Ríos and Emilio Botín-Sanz de Sautuola López, respectively.

Santander has been designated as a Significant Institution since the entry into force of European Banking Supervision in late 2014, and as a consequence is directly supervised by the European Central Bank.

Banco Santander was founded in 1857 as Banco de Santander. In 1999, it merged with Banco Central Hispano, or BCH, which had in turn been formed through the 1991 merger of Banco Central and Banco Hispanoamericano. The combined bank, known as Banco Santander Central Hispano, or BSCH, was designed to be a "merger of equals", in which the top executives of the two pre-existing firms would share control of the merged entity.

Soon after the merger former BCH executives accused Banco Santander chairman Emilio Botín Sanz de Sautuola y García de los Ríos of trying to push his own agenda and threatened to take legal action. This post-merger disagreement was resolved when BCH executives Jose Amusátegui and Angel Corcóstegui agreed to accept severance payments, retire and pass control to Botín, at an expense to shareholders of €183 million. The large termination payouts generated negative press, and Botín was eventually brought to trial on criminal charges of "misappropriation of funds" and "irresponsible management". However, in April 2005, the court cleared him of all charges, the €164 million retirement payments made to the two former executives having been found to be legal, "made as compensation for the services provided to the bank". Also that year, the anti-corruption division of the Spanish public prosecutor's office cleared Botín of all charges in a separate case, in which he was accused of insider trading.

In 2007, the bank officially changed the official name back to Banco Santander S.A.

In 1996, Banco Santander acquired Grupo Financiero InverMexico. In 2000, Banco Santander Central Hispano acquired Grupo Financiero Serfin of Mexico.