Niterói is a municipality of the state of Rio de Janeiro in the southeast region of Brazil. It lies across Guanabara Bay from the city of Rio de Janeiro and forms part of the Rio de Janeiro Metropolitan Area. It has an estimated population of 487,327 inhabitants (2010) and an area of , making it the sixth most populous city in the state. It has the highest Human Development Index of the state. The city has the nicknames of Nikity, Nicki City and the Smile City (Cidade Sorriso).
Studies by the Getulio Vargas Foundation in June 2011 classified Niterói as the richest city of Brazil, with 55.7% of the population included in class A. Considering the classes A and B, Niterói also appears in the first place, with 85.9% of the population entered in these classes.
The word "Niterói" comes from the Tupi language and means "water that hides". It was founded on 22 November 1573 by the Tupi Amerindian chief Araribóia (who later was converted to Roman Catholicism and given the Christian name of Martim Afonso, after the Portuguese explorer Martim Afonso de Sousa). It makes Niteroi the only Brazilian city to have been founded by a non-Christian, non-assimilated Brazilian Amerindian.
The Brazilian leg of the Red Bull Cliff Diving World Series takes place in Niterói.