Guanabara Bay (Portuguese: baía de Guanabara, baía da Guanabara, IPA: [baˈi.ɐ da ɡwɐnaˈbaɾɐ]) is an oceanic bay in Southeast Brazil in the state of Rio de Janeiro. On its western shore lie the cities of Rio de Janeiro and Duque de Caxias, and on its eastern shore are 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 (after the All Saints' Bay), at 412 square kilometres (159 sq mi), with a perimeter of 143 kilometres (89 mi).
Guanabara Bay is 31 kilometres (19 mi) long and 28 kilometres (17 mi) wide at its maximum. Its 1.5 kilometres (0.93 mi) wide mouth is flanked at the eastern tip by the Pico do Papagaio (Parrot's Peak) and the western tip by Pão de Açúcar (Sugar Loaf).