Municipality of Kostajnica is situated in the northwest part of Bosnia and Herzegovina, on the right riverbank of Una River. Una River makes its north and northwest border, and at the same time makes the state borderline with Republic of Croatia. In the east, Kostajnica lies on the border with municipality of Kozarska Dubica, and in the south with municipality of Novi Grad. In the territorial sense, the territory of the Kostajnica municipality belongs to the area that is also known as Western Potkozarje i.e. Prijedor region. The area of the municipality is circa 132 km2. There are 11 villages and the centre of the Municipality-Town of Kostajnica within the area of the municipality.
In its history, the municipality of Kostajnica had much bigger territory, up to Bosanski Novi i.e. up to residential area Cerovica and parallel with Sana River course including residential areas Prusci, Jutrogusta, Kozara slopes all the way to Mljecanica River and down its course to Una River. The territory changed during times and until 1962, apart from the current 11 residential area, the area of the municipality also included residential areas Strigova, Citluk, Donja Slabinja, Babinjac and Bacvani which are today part of the Kozarska Dubica Municipality. Since then, the area of our municipality has consisted of the following residential areas: Petrinja, Tavija, Mrakodol, Grdanovac, Pobrdjani, Kalenderi, Mraovo Polje, Zovik, Gumnjani, Podoska and Gornja Slabinja.