Niterói is a municipality in the state of Rio de Janeiro, in the southeast region of Brazil. It lies across Guanabara Bay, facing the city of Rio de Janeiro and forming part of the Rio de Janeiro Metropolitan Area. It was the capital of Rio de Janeiro, as marked by its golden mural crown, from 1834 to 1894 and again from 1903 to 1975. It has an estimated population of 515,317 inhabitants (2020) and an area of 129.375 km2 (49.952 sq mi), making it the fifth most populous city in the state. It has the highest Human Development Index in the state and the seventh highest among Brazil's municipalities in 2010. Individually, it is the second municipality with the highest average monthly household income per capita in Brazil and appears in 13th place among the municipalities of the country according to social indicators related to education. The city has the nicknames of Cidade Sorriso.

Rio de Janeiro is one of the 27 federative units of Brazil. It has the second largest economy of Brazil, with the largest being that of the state of São Paulo. The state, which has 8.2% of the Brazilian population, is responsible for 9.2% of the Brazilian GDP.