Municipality of Kostajnica is located in the northwest part of Republic of Srpska. It is situated between Novi Grad and Kozarska Dubica. Its area is about 130 km2 and has populationof about 6000. Una River flows through Kostajnica, as one of the most significant natural resources.
Its altitude is between 110 and 407 metres.
In addition to Una River, one of the most important characteristics of this municipality is the richness in forests and chestnut trees. There has been the traditional “Chestnut Festival” in Kostajnica for many years, as one of the most important events Kostajnica municipality is recognizable for.