The 5 October 1910 revolution was the overthrow of the centuries-old Portuguese monarchy and its replacement by the First Portuguese Republic. It was the result of a coup d'état organized by the Portuguese Republican Party.

By 1910, the Kingdom of Portugal was in deep crisis: national anger over the 1890 British Ultimatum, the royal family's expenses, the assassination of the King and his heir in 1908, changing religious and social views, instability of the two political parties (Progressive and Regenerador), the dictatorship of João Franco, and the regime's apparent inability to adapt to modern times all led to widespread resentment against the Monarchy. The proponents of the republic, particularly the Republican Party, found ways to take advantage of the situation. The Republican Party presented itself as the only one that had a programme that was capable of returning to the country its lost status and place Portugal on the way of progress.

After a reluctance of the military to combat the nearly two thousand soldiers and sailors that rebelled between 3 and 4 October 1910, the Republic was proclaimed at 9 o'clock a.m of the next day from the balcony of the Lisbon City Hall in Lisbon. After the revolution, a provisional government led by Teófilo Braga directed the fate of the country until the approval of the Constitution in 1911 that marked the beginning of the First Republic. Among other things, with the establishment of the republic, national symbols were changed: the national anthem and the flag. The revolution produced some civil and religious liberties.