Alfonso Carlos, Infante of Spain, Duke of San Jaime (Alfonso Carlos Fernando José Juan Pío; London, 12 September 1849 – Vienna, 29 September 1936) was the Carlist claimant to the throne of Spain under the name Alfonso Carlos I (though some Carlists who supported Alfonso XIII as his heir later referred to him as Alfonso XII) and the Legitimist claimant to the throne of France under the name Charles XII.

Alfonso Carlos was the second son of Infante Juan of Spain, Count of Montizón and Archduchess Maria Beatrix of Austria-Este. Since his parents separated when he was young, he and his elder brother were raised in Modena under the tutelage of his maternal uncle Duke Francis V of Modena.

In 1868 Alfonso Carlos joined the Papal Zouaves which had been formed to defend the Papal States from the army of the Kingdom of Italy. In 1869 he was promoted to the rank of lieutenant. In September 1870 he fought for the pope during the Capture of Rome where he defended the Porta Pia. When he was ordered to surrender, he refused to give up his sword which had belonged to his grandfather Carlos V. He escaped to Toulon in a French naval ship.

In 1872 Alfonso Carlos joined the armies of his older brother Carlos, Duke of Madrid, in the Third Carlist War in Spain. He was appointed commanding general of the Royal Army of Catalonia and distinguished himself at the Battle of Alpens in July 1873 and the siege of Cuenca in July 1874. In spite of these successes, however, the Carlists eventually lost the war which was over by February 1876.

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