José Plácido de Castro was a Brazilian soldier, surveyor, rubber producer and politician who led the armed revolt during the Acre War of 1902–3, when the Republic of Acre broke away from Bolivia. He was the president of the state of Acre just before and after it was purchased by Brazil. After the war he became extremely wealthy as a rubber producer, and made many enemies. He was assassinated in 1908. He is considered a hero of Brazil.

The Acre War, known in Brazil as Acrean Revolution and in Spanish as Guerra del Acre was a border conflict between Bolivia and Brazil over the Acre Region, which was rich in rubber and gold deposits. The conflict had two phases between 1899 and 1903 and ended with an Acrean victory and the subsequent Treaty of Petrópolis, which ceded Acre to Brazil. The outcome also affected territories disputed with Peru.