Guanabara Bay is an oceanic bay located in Southeast Brazil in the state of Rio de Janeiro. On its western shore lies the cities of Rio de Janeiro and Duque de Caxias, and on its eastern shore the cities of Niterói and São Gonçalo. Four other municipalities surround the bay's shores. Guanabara Bay is the second largest bay in area in Brazil, at 412 square kilometres, with a perimeter of 143 kilometres.
Guanabara Bay is 31 kilometres long and 28 kilometres wide at its maximum. Its 1.5 kilometres wide mouth is flanked at the eastern tip by the Pico do Papagaio and the western tip by Pão de Açúcar.
The name Guanabara comes from the Tupi language, goanã-pará, from gwa "bay", plus nã "similar to" and ba'ra "sea". Traditionally, it is also translated as "the bosom of sea".