Amadeo I (sometimes Latinized as Amadeus) (May 30, 1845 – January 18, 1890), Duke of Aosta and King of Spain, was born in Turin, Italy. He was the second son of Victor Emmanuel II (King of Piedmont, Savoy, Sardinia and, later, first King of Italy) and Mary Adelaide of Austria.

In 1867, he married Maria Victoria al Pozzo della Cisterna. One year later, after the Spanish revolution deposed Isabella II, the new Cortes decided to reinstate a monarchy. Amadeus I was elected King on November 16, 1870. He swore to uphold the constitution in Madrid on January 2, 1871.

The election of the new King coincided with the assassination of General Juan Prim, his main backer. After that, Amadeus had to deal with difficult situations, with unstable Spanish politics, republican conspiracies, Carlist uprisings, separatism in Cuba, same-party disputes, fugitive governments and assassination attempts.

He could only count on the support of the progressive party, whose leaders were trading off in the government thanks to parliamentary majority and electoral fraud. The progressives divided into monarchists and constitutionalists, which made the instability worse, and in 1872 a violent outburst of interparty conflicts hit a peak. The Carlists uprose in the Basque and Catalan regions, and after that, republican uprisings happened in various cities dispersed across the country. The artillery corps of the army went on strike, and the government instructed the King to discipline them.

With the possibility of reigning with no popular support, Amadeus issued an order against the artillery and then immediately abdicated from the Spanish throne on February 11, 1873. At ten o'clock that same night, Spain was proclaimed a republic, at which Amadeus made an appearance before the Cortes, proclaiming the Spanish people ungovernable.

Completely disgusted, the ex-monarch left Spain to go back to Italy, where he took over the title of Duke of Aosta. There he stayed until he died.

The first Spanish Republic lasted less than two years, and in 1875 Alfonso XII, the son of Isabella II, was proclaimed king, with Antonio Cánovas del Castillo, Spanish prime minister from 1873 until his assassination in 1893, briefly regent.

Seventy years later, Amadeus's grandson, Aimone of Aosta was proclaimed king of the Independent State of Croatia as Tomislav II. He never actually reigned and only lasted 29 months before abdicating.