Juan José Castelli (July 19, 1764 – October 12, 1812) was an Argentine lawyer.
He was one of the leaders of the May Revolution, which started the Argentine War of Independence.
He led an ill-fated military campaign in Upper Peru.
Juan José Castelli was born in Buenos Aires, and went to school at the Real Colegio de San Carlos in Buenos Aires and Monserrat College in the city of Córdoba, Argentina.
He graduated as a lawyer from the University of Charcas, in Upper Peru.
His cousin, Manuel Belgrano, introduced him to the public administration of the Viceroyalty of the Rio de la Plata.
Along with Belgrano, Nicolás Rodríguez Peña, and Hipólito Vieytes, Castelli planned a revolution to replace the absolute monarchy with the new ideas of the Age of Enlightenment.
He led the Buenos Aires patriots during the May Revolution, which ended with the removal of viceroy Baltasar Hidalgo de Cisneros from power.
He is known as the "Speaker of the Revolution" for his speech during the open cabildo held in Buenos Aires on May 22, 1810.
Castelli was appointed a Committee member of the Primera Junta and was sent to Córdoba to end Santiago de Liniers's counter-revolution.
He succeeded, and ordered the execution of Liniers and his supporters.
He then commanded the establishment of a revolutionary government in Upper Peru (today's Bolivia) with the aim of freeing the indigenous peoples and African slaves.
In 1811 Castelli signed a truce with the Spanish in Upper Peru, but they betrayed him and caught the Northern Army unprepared.
As a result, the Argentines suffered a major loss in the Battle of Huaqui on June 20, 1811.
When Castelli returned to Buenos Aires, the First Triumvirate imprisoned him for losing the battle, and Castelli died shortly afterwards from tongue cancer.