Ilha do Presídio, Historical islet in Guaíba Lake, Porto Alegre, Brazil.

Ilha do Presídio is an islet in Guaíba Lake in Porto Alegre featuring two main structures: the Guard Building and the Prison Building with military design elements. The structures reflect functional design built for maximum security purposes.

Built between 1857 and 1860, the structures first housed Porto Alegre's Fourth Gunpowder House, used by the Army until 1930. From 1940 the state administered the islet, running an animal research laboratory focused on swine fever in 1947 and 1948. In 1950 it became a prison, and under the military dictatorship it held political prisoners; it was deactivated in 1983.

The prison held political prisoners during Brazil's military dictatorship, among them Sergeant Manoel Raymundo Soares, arrested by the DOPS and found dead in Guaíba Lake five months later with his hands tied behind his back. The widely reported "Caso das Mãos Amarradas" gave the site its place in the memory of the dictatorship years.

The islet is not open to regular visitors, but its structures can be viewed from boat tours departing from Porto Alegre's waterfront. It is best to check for available tours before heading to the harbor area.

The islet has carried three different names throughout history: Ilha do Presídio, Ilha das Pedras Brancas, and Ilha da Pólvora, each reflecting its changing purpose. Each name tells a chapter of the site's story.