Oulujärvi (Шаблон:Pron, svenska. Ule träsk) is a lake in Finland located in the Oulu Province. With the area of 887 km² it is the fifth largest lake in the country. The lake drained by the Oulu River, which flows northwestward from the lake to the Gulf of Bothnia. Its nickname is the "Kainuu sea", and it is surrounded by three municipalities: Vaala, Paltamo and Kajaani. About 40 percent of the lake is in the Vaala municipality.