Events in the year 1866 in Uruguay.
Incumbents President: Venancio Flores
Events May 2 - Paraguayan War: Battle of Estero Bellaco July 18 - Battle of Boquerón (1866) September 8 - establishment of Nuestra Señora del Carmen, Aguada, Montevideo September 22 - Battle of Curupayty Deaths July 18 - León de Pallejas, Spanish-born general, killed in action