Guanabara, former state, SE Brazil, on the Atlantic Ocean; since 1974, part of the state of Rio de Janeiro. Guanabara Bay, a deep inlet of the Atlantic Ocean, is noted for its beauty. The site of the city of Rio de Janeiro, it is also called Rio de Janeiro Bay.

The State of Guanabara (in Portuguese: Estado da Guanabara), was the name given to the municipality of Rio de Janeiro (now the city with the same name), after the moving of the national capital to Brasília in 1960. It was named after the Guanabara Bay, which borders its Eastern coast.

In 1834, the city of Rio de Janeiro became a municipality and was elevated to imperial capital. With the proclamation of the republic in 1889, the city of Rio de Janeiro became the Federal District, and the province of Rio de Janeiro became a State with Niterói as the State capital. With the moving of the national capital to Brasília in 1960, the municipality of Rio de Janeiro became the State of Guanabara.

In 1975, the states of Guanabara and Rio de Janeiro merged under the name of Rio de Janeiro, with the city of Rio de Janeiro as state capital.