Oulujärvi (Finnish pronunciation: [ˈouluˌjærʋi], Swedish: Ule träsk) is a large lake in Finland located in the Kainuu. With the area of 928 square kilometres (358 sq mi) 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.