Fort Prince of Beira, Portuguese colonial fort in Costa Marques, Brazil

Forte Príncipe da Beira is a colonial military complex in Costa Marques on the banks of the Guaporé River, near the border with Bolivia. The fortification consists of four angular corner bastions linked by thick stone walls that enclose a large central courtyard, once used as a parade ground and assembly point for the garrison.

Construction began on 19 April 1775, and the foundation stone was laid on 20 June 1776 under Luís de Albuquerque de Melo Pereira e Cáceres, governor of Mato Grosso, who chose the right bank of the Guaporé to replace the flood-damaged Forte de Bragança. The fort was named for D. José de Bragança, then Prince of Beira, heir presumptive to the Portuguese crown. It was built from 1776 to 1783 and abandoned for military use in 1889.

The fort is named after Dom José de Bragança, heir to the Portuguese throne, who held the title Prince of Beira until his mother Maria I ascended in 1777. A quilombola community certified by the Palmares Foundation in 2005 lives at the site today.

The site lies several kilometers outside Costa Marques and is reached by an unpaved road that can become difficult to navigate during heavy rains. Those exploring the walls and bastions should wear sturdy footwear, as the ground is uneven in places and overgrown with vegetation.

The complex was one of the few Portuguese fortifications built far from the Atlantic coast in Brazil's interior. Construction required transporting heavy stones and brickloads over hundreds of kilometers through dense rainforest and along the river, a logistical achievement for the time.