Alfonso XIII (Spanish: Alfonso León Fernando María Jaime Isidro Pascual Antonio de Borbón y Habsburgo-Lorena; French: Alphonse Léon Ferdinand Marie Jacques Isidore Pascal Antoine de Bourbon; 17 May 1886 – 28 February 1941), also known as El Africano or the African due to his Africanist views, was King of Spain from his birth until 14 April 1931, when the Second Spanish Republic was proclaimed. He was a monarch from birth as his father, Alfonso XII, had died the previous year. Alfonso's mother, Maria Christina of Austria, served as regent until he assumed full powers on his sixteenth birthday in 1902.

Alfonso XIII's upbringing and public image were closely linked to the military estate, often presenting himself as a soldier-king. His effective reign started four years after the so-called 1898 Disaster, with various social factions projecting their expectations of national regeneration upon him. Similarly to other European monarchs of his time, he played an important political role, entailing a highly controversial use of his constitutional executive powers. His wedding with Victoria Eugenie of Battenberg in 1906 was marked by a regicide attempt, from which he escaped unharmed.

With a divided opinion of the public eye on World War I, and moreover a split between the pro-German and pro-Entente sympathizers, Alfonso XIII leveraged his family relations to every major European royal family to help preserve the stance of neutrality espoused by the government. Several factors led to undermine the monarch's constitutional legitimacy: the rupture of the turno system, the further deepening of the Restoration system crisis in the 1910s, a trio of crises in 1917, the spiral of violence in Morocco, and the lead up to the installment of the dictatorship of Miguel Primo de Rivera through a 1923 military coup d'état that won the acquiescence from Alfonso XIII.

Upon the political failure of the dictatorship, Alfonso XIII removed support from Primo de Rivera (who was thereby forced to resign in 1930) and favoured (during the so-called dictablanda) a return to the pre-1923 state of affairs. Nevertheless, he had lost most of his political capital along the way. He left Spain voluntarily after the municipal elections of April 1931 – which was understood as a plebiscite on maintaining the monarchy or declaring a republic – the result of which led to the proclamation of the Second Spanish Republic on 14 April 1931.

His efforts with the European War Office during World War I earned him a nomination on the Nobel Peace Prize in 1917, which was ultimately won by the Red Cross. To date, he remains the only monarch to have been nominated for a Nobel Prize.