The Thyssen-Bornemisza National Museum (Spanish: Museo Nacional Thyssen-Bornemisza, pronounced [muˈseo ˈtisen boɾneˈmisa]; named after its founder, Baron Heinrich Thyssen-Bornemisza), or simply the Thyssen, is an art museum in Madrid, Spain, located near the Prado Museum on one of the city's main boulevards. It is known as part of the "Golden Triangle of Art", which also includes the Prado and the Reina Sofía national galleries. The Thyssen-Bornemisza fills the historical gaps in its counterparts' collections: in the Prado's case this includes Italian primitives and works from the English, Dutch and German schools, while in the case of the Reina Sofía it concerns Impressionists, Expressionists, and European and American paintings from the 20th century.

With over 1,600 paintings, it was once the second largest private collection in the world after the British Royal Collection. A competition was held to house the core of the collection in 1987–88 after Baron Thyssen, having unsuccessfully sought permission to enlarge his museum in Lugano (Villa Favorita), searched for a better-suited location elsewhere in Europe.

The collection was started in the 1920s as a private collection by Heinrich, Baron Thyssen-Bornemisza de Kászon. In a reversal of the movement of European paintings to the US during this period, one of the elder Baron's sources was the collections of American millionaires coping with the Great Depression and inheritance taxes. In this way he acquired old master paintings such as Ghirlandaio's portrait of Giovanna Tornabuoni (once in the Morgan Library) and Carpaccio's Knight (from the collection of Otto Kahn). The collection was later expanded by Heinrich's son Baron Hans Heinrich Thyssen-Bornemisza (1921–2002), who assembled most of the works from his relatives' collections and proceeded to acquire large numbers of new works (from Gothic art to Lucian Freud).

The collection was initially housed in the family estate in Lugano in a twenty-room building modelled after the Neue Pinakothek in Munich. In 1988, the Baron filed a request for building a further extension designed by British architects James Stirling and Michael Wilford, but the plan was rejected by the Lugano City Council.

In 1985, the Baron married Carmen "Tita" Cervera (a former Miss Spain 1961) and introduced her to art collecting. Cervera's influence was decisive in persuading the Baron to relocate the core of his collection to Spain where the local government had a building available next to the Prado. The Thyssen-Bornemisza Museum officially opened in 1992, under the directorship of Tomás Llorens, showing 715 works of art. A year later, the Spanish Government bought 775 works for $350 million. These pieces are now in the purpose-built museum in Madrid. After the museum opened, in 1999, Cervera loaned 429 works of her own art collection to the museum for 11 years. The loan was renewed annually for free from 2012 to 2021.

The Baroness remains involved with the museum. She personally decided the salmon pink tone of the interior walls and in May 2006, publicly demonstrated against plans of the Mayor of Madrid, Alberto Ruiz-Gallardón to redevelop the Paseo del Prado as she thought the works and traffic would damage the collection and the museum's appearance.

In 2015, the Baroness delayed the annual renewal of her loan while deciding whether or not to temporarily move her collection for a fee to a museum in Barcelona, the United States, or Russia. She eventually decided to keep the collection in Madrid, but in 2017, she again delayed signing the agreement. In 2021, the Ministry of Culture officially finalized an agreement to loan the collection for an annual fee of 6.5 million euros ($7.8 million) over the course of 15 years.

The Old Masters were mainly bought by the elder Baron, while Hans focused more on the 19th and 20th century, resulting in a collection that spans eight centuries of European painting, without claiming to give an all-encompassing view but rather a series of highlights.