Oulujärvi (, ) is a large lake in Finland located in the Kainuu region. With an area of it is the fifth largest lake in the country. The lake is drained by the Oulu River, which flows northwestward from the lake into the Gulf of Bothnia. Its nickname is the "Kainuu sea", and it is bordered by three municipalities: Vaala, Paltamo and Kajaani. About 40 percent of the lake is in the Vaala municipality.