The Gacka is a river located in the Lika region of central Croatia.

Because for a large part of its course it is a subterranean river, estimates of its length vary. The aboveground part has undergone substantial human intervention - before it was 32 km long; now it is only 11 km long. It has been regulated by man to flow into an aqueduct in Gusic polje (known as Tunel Gacka-Gusic Jezero) and