The Naval Battle of the Riachuelo is fought on the rivulet Riachuelo (Argentina), between the Paraguayan Navy on one side and the Brazilian Navy on the other. The Brazilian victory was crucial for the later success of the Triple Alliance (Brazil, Uruguay, and Argentina) in the Paraguayan War.

The Battle of the Riachuelo was a large and decisive naval battle of the Paraguayan War between Paraguay and the Empire of Brazil. By late 1864, Paraguay had scored a series of victories in the war, but on June 11, 1865, its naval defeat by the Brazilians on the Paraná River began to turn the tide in favor of the allies.This was the largest naval battle fought between two South American countries.