| Infante Alfonso Carlos | |
|---|---|
| Duke of San Jaime | |
 The Duke of San Jaime in 1877 | |
| Carlist pretender to the Spanish throne as Alfonso Carlos I | |
| Pretence | 2 October 1931 – 29 September 1936 |
| Predecessor | Jaime III |
| Successor | Carlos VIII or Javier I |
| Legitimist pretender to the French throne | |
| Pretence | 2 October 1931 – 29 September 1936 |
| Predecessor | Infante Jaime, Duke of Madrid |
| Successor | Alfonso XIII of Spain |
| Born | 12 September 1849 London, England |
| Died | 29 September 1936 (aged 87) Vienna, Austria |
| Burial | |
| Spouse | Infanta Maria das Neves of Portugal |
| House | Bourbon |
| Father | Infante Juan, Count of Montizón |
| Mother | Maria Beatrix of Austria-Este |
| Signature |  |
Infante Alfonso Carlos of Spain, Duke of San Jaime (Spanish: Alfonso Carlos Fernando José Juan Pío; French: Alphonse Charles Ferdinand Joseph Jean Pieux; 12 September 1849 in London – 29 September 1936 in Vienna) was the Carlist claimant to the throne of Spain under the name Alfonso Carlos I; some French Legitimists declared him also the king of France as Charles XII, though Alfonso never officially endorsed these claims.
In 1870 and in the ranks of the papal troops, he defended Rome against the Italian Army. In 1872–1874, he commanded sections of the front during the Third Carlist War. Between the mid-1870s and the early 1930s, he remained withdrawn into privacy, living in his residences in Austria. His public engagements were related to the buildup of an international league against dueling.
Upon the unexpected death of his nephew Infante Jaime, Duke of Madrid in 1931, he inherited the Spanish and French monarchical claims. As an octagenarian he dedicated himself to development of Carlist structures in Spain. He led the movement into the anti-Republican conspiracy, which resulted in Carlist participation in the July coup d'état. As he had no children, Alfonso Carlos was the last undisputable Carlist pretender to the throne; after his death the movement was fragmented into branches supporting various candidates.