The Kajaani Castle (, , Kajaneborgs slott, older spelling Cajanaborg) is a ruined fortress built of granite in Kajaani. The castle was built in the early 17th century on an islet in the Kajaani River, between the Ämmäkoski and Koivukoski rapids in Kainuu, present-day Finland. It was the last medieval-style stone castle constructed in Sweden.

The castle was built at a strategically valuable site