Juan Carlos I (/ˌhwɑːŋˈkɑːrloʊs/, Spanish: [xwaŋˈkaɾlos]; Juan Carlos Alfonso Víctor María de Borbón y Borbón-Dos Sicilias, born 5 January 1938) reigned as King of Spain from 1975 to 2014, when he abdicated in favour of his son, Felipe VI.

Generalísimo Francisco Franco, the Spanish dictator, took over the government of Spain from the short-lived Second Spanish Republic following victory in the Spanish Civil War in 1939, and ruled as "Regent to the [exiled] King of Spain". In 1969, he chose Juan Carlos, grandson of King Alfonso XIII, to be the next head of state, bypassing his father Juan de Borbón and expecting him to continue Franco's own authoritarian regime. Juan Carlos became king on 22 November 1975, two days after Franco's death, the first reigning monarch since 1931; although his exiled father did not officially abdicate in favor of his son until 1977. Soon after enthronement, however, Juan Carlos introduced reforms to dismantle the Francoist regime and begin the Spanish transition to democracy. This led to the approval of the Spanish Constitution of 1978 in a referendum, which re-established a constitutional monarchy. In 1981, Juan Carlos played a major role in preventing a coup that attempted to revert Spain to Francoist government in the King's name.

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