Oulujärvi or Lake Oulu or Ule, is a large lake in the Kainuu region of Finland. It is also historically known as Lake Cajania from a former name of Kainuu. With an area of 928 km2 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 the three municipalities of Vaala, Paltamo, and Kajaani. About 40 percent of the lake is in Vaala.