Júlio de Castilhos Museum (Portuguese: Museu Júlio de Castilhos) is a museum located in the city of Porto Alegre (Rio Grande do Sul, Brazil). It is the oldest museum in Rio Grande do Sul state.
Its collection, over 10 thousand pieces, is Brazilian national heritage, and comprises objects of historical, artistic, ethnographic and archaeological character, mostly related to the history of Rio Grande do Sul. Its sections for the War of Tatters and the War of the Triple Alliance are specially rich. The museum also preserves a few but very important examples of sculptures produced within Indian Reductions by Jesuits and indigenous peoples.