São Sebastião is a Brazilian municipality, located on the southeast coast of Brazil, in the state of São Paulo. The population in 2020 was 90,328, its density was 185/km² and the area is 403 km². The Tropic of Capricorn lies 25 km north. The municipality existed since 1636 and formed a part of the old hereditary captaincy of Santo Amaro.
The archipelago municipality of Ilhabela is located on the east coast of the city; the largest island of the archipelago is also called São Sebastião. Between the city and the island, there is the São Sebastião channel with 30 kilometres in length, and variable width. There is an oil terminal at the channel, owned by Transpetro, a subsidiary of Petrobrás.
The city is famous for its beaches, which makes it a popular tourism destination, especially for people from the state of São Paulo. Near the boundary with Bertioga, there is a small Guarani village managed by FUNAI.