Amadeo (Italian: Amedeo, sometimes latinized as Amadeus; 30 May 1845 – 18 January 1890) was an Italian prince who reigned as King of Spain from 1870 to 1873. The first and only King of Spain from the House of Savoy, he was the second son of King Vittorio Emanuele II of Italy and was known for most of his life as the Duke of Aosta, the usual title for a Savoy second son.

He was elected by the Cortes as Spain's monarch in 1870, following the deposition of Isabel II, and was sworn in the following year. Amadeo's reign was fraught with growing republicanism, Carlist rebellions in the north, and the Cuban independence movement. He abdicated and returned to Italy in 1873, and the First Spanish Republic was declared as a result.

Granted the hereditary title of Duke of Aosta in the year of his birth, he founded the Aosta branch of Italy's royal House of Savoy, which is junior in agnatic descent to the branch descended from King Umberto I that reigned in Italy until 1900, but senior to the branch of the dukes of Genoa.