Kostajnica (Serbian Cyrillic: Костајница), formerly Bosanska Kostajnica (Serbian Cyrillic: Босанска Костајница), is a town and municipality located in Republika Srpska, Bosnia and Herzegovina. It is situated in the Кrajina region. As of 2013, the municipality had a population of 5,977 inhabitants, while the town of Kostajnica has a population of 4,047 inhabitants. The municipality was created from a part of the pre-war municipality of Novi Grad. It is located across the Una river from Hrvatska Kostajnica, Croatia.