Forte Principe da Beira (Portuguese for "Prince of Beira Fortress") is a fortification on the lower part of the Guaporé River close to its confluence with the Mamoré River in the Brazilian state of Rondônia.

It was named after the heir of Portugal, styled as the Prince of Beira.

The Portuguese built it to secure their border against the expanding Spanish Empire, which controlled the areas to the southwest of Rondônia.

Inside the wall there are ruins of a chapel, soldier quarters, a prison and various utility buildings.

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