Amadeo I (Italian: Amedeo Ferdinando Maria di Savoia; 30 May 1845 – 18 January 1890), also known as Amadeus, was an Italian prince who reigned as King of Spain from 1870 to 1873. The only king of Spain to come from the House of Savoy, he was the second son of Victor Emmanuel II of Italy and was known for most of his life as the Duke of Aosta, the usual title for a second son in the Savoyard dynasty.

He was elected by the Cortes Generales 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. After three tumultuous years on the throne, he abdicated and returned to Italy in 1873, and the First Spanish Republic was declared as a result.

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 1946, but senior to the branch of the dukes of Genoa.

Prince Amadeo of Savoy was born in Turin, then part of the Kingdom of Sardinia. He was the third child and second son of King Victor Emmanuel II, who would later become the first King of a unified Italy, and of Archduchess Adelaide of Austria. He was granted the hereditary title of Duke of Aosta from birth.

Entering the Royal Sardinian Army as captain in 1859, he fought through the Third Italian War of Independence in 1866 with the rank of major-general. He led his brigade into action at the Battle of Custoza and was wounded at Monte Croce. In 1868, after his marriage, he was made vice admiral of the Italian Royal Navy, but the position ended when he ascended the Spanish throne.

In 1867, his father yielded to the entreaties of the parliamentary deputy Francesco Cassins, and on 30 May of that year, Amedeo was married to Donna Maria Vittoria dal Pozzo. The King initially opposed the match on the grounds that her family was of insufficient rank and that he hoped for his son to marry a German princess. Despite her princely title, Donna Maria Vittoria was not of royal birth and belonged rather to the Piedmontese nobility. She was, however, the sole heir to her father's vast fortune, which subsequent Dukes of Aosta inherited, thereby obtaining wealth independent of their dynastic appanage and allowances from Italy's kings. The wedding day of Prince Amedeo and Donna Maria Vittoria was marred by the death of a station master, who was crushed under the wheels of the honeymoon train.

In March 1870, Maria Vittoria appealed to the King to remonstrate with her husband for marital infidelities, which caused her hurt and embarrassment. However, the King wrote in reply that he understood her feelings, but he considered that she had no right to dictate her husband's behaviour, and her jealousy was unbecoming.

After the Glorious Revolution deposed Isabella II in September 1868, the new Cortes began the task of searching for a suitable liberal-leaning candidate from a new dynasty to replace her. Eventually the Duke of Aosta was taken into consideration. His father was a descendant of King Philip II of Spain through his daughter Infanta Catalina Micaela of Spain and her son Thomas Francis, Prince of Carignano, while his mother was a descendant of King Charles III of Spain through his daughter Infanta Maria Luisa of Spain. The Savoyard prince was elected king as Amadeo I on 16 November 1870 and swore to uphold the Constitution in Madrid on 2 January 1871. While the new king was on his way to Spain, General Juan Prim, his chief supporter, was assassinated and Amadeo took the oath in the presence of Prim's corpse.