Parnaíba (Portuguese: [paʁnaˈibɐ]) is a city in the Brazilian state of Piauí. Having a population of over 150,000 inhabitants according to IBGE's 2020 report, it is the second most populous city in the state, after the capital Teresina. It is one of the four coastal municipalities in Piauí (besides Ilha Grande, Luís Correia, and Cajueiro da Praia).

Besides the natural environment, Parnaíba has historical value for Piauí; near Porto das Barcas several historic buildings reflect the importance of Parnaíba, coming at that time to be more important than the former capital Oeiras and taking up same references in Europe.