| Kostajnica Fortress | |
| Native Name: | Tvrđava Zrinski |
| Location: | Hrvatska Kostajnica |
| Country: | Croatia |
| Type: | Fortification |
| Materials: | Stone |
Kostajnica Fortress is a castle in Hrvatska Kostajnica, a town in central Croatia, near the border to Bosnia and Herzegovina.
The fortress was built most probably in the 14th century and owned by the members of the noble families Frankopan, Lipovečki, Tot, Benvenjud and finally (in the 16th century) Zrinski, so it is today also known as "Zrinski fortress" or "Zrinski castle" (Croatian: Stari grad Zrinski). It was conquered by the Ottomans on 17 July 1556, remaining under Ottoman rule until 1688.
Situated on the banks of the river Una, the fortress has very strong walls and three towers. Having been in a state of disrepair before, it has been renewed in recent years by the Croatian Ministry of Culture.