Portugal was formerly a kingdom and became a republic in 1910. The change occurred during the 5 October 1910 Revolution, which overthrew the Portuguese monarchy and established the First Portuguese Republic.
The revolution followed years of political and social tension. Republican opposition grew after Portugal’s humiliation in the 1890 British Ultimatum, which forced the country to abandon territorial ambitions in Africa. Public distrust also deepened after financial problems, political instability, the authoritarian government of João Franco, and the 1908 assassination of King Carlos I and his heir, Crown Prince Luís Filipe.
The uprising began in Lisbon on 3 October 1910. Republican forces gained control, and the republic was proclaimed from Lisbon City Hall on 5 October. King Manuel II, Portugal’s final monarch, went into exile in Britain. A provisional government led the country until a republican constitution was approved in 1911.
The date is sometimes confused with 1911 because the constitution and formal institutional organization of the First Republic followed later. The monarchy itself ended in 1910, making 1910 the canonical answer.