The Flag of the Treinta y Tres pays homage to the military expedition of the Treinta y Tres Orientales (), a militant revolutionary group led by Juan Antonio Lavalleja and Manuel Oribe which fought against the Empire of Brazil during the Brazilian occupation of the Provincia Oriental. The flag was first used during the disembarkation of the Thirty-Three Orientals in Uruguayan territory the 19th of April, 1825, at the beginning of the Cisplatine War.