Hrvatska Kostajnica, also referred to as simply Kostajnica, is a town in central Croatia located in the Sisak-Moslavina County. It sits on the Una river, positioned south of both Petrinja and Sisak. The town is situated directly across the river from Kostajnica in Bosnia and Herzegovina, making it part of a transnational river region.
The town's name likely derives from the word Kostanj, meaning chestnut in Croatian. Historically, Hrvatska Kostajnica was known by various names including Koztainicha, Kaztanicha, and Costgnanica, reflecting the linguistic variations of earlier periods.