The Guaíba River (Portuguese pronunciation: [ɡwaˈiba]) is a waterway in Rio Grande do Sul, a state in southern Brazil. It is famous for its beautiful reflection at sunset. The Guaíba is an extension of the Lagoa dos Patos, a huge coastal lagoon to the south, and although commonly referred to as a "river", the Guaíba is an estuary, and is also referred to as a bay, lake, or lagoon. The Jacuí River empties into the Guaíba from the north.
Porto Alegre, the capital of Rio Grande do Sul, lies on the eastern shore of the Guaíba River, and the city of Guaíba lies opposite Porto Alegre on the western shore. The Guaíba is navigable, and is connected to the Atlantic Ocean via the Lagoa dos Patos and its outlet channel, located farther south at Rio Grande.