The Tržan Castle (; or Utvrda Tržan) is a ruined medieval castle situated above the village of Modruš in the northern part of historical Lika region, central Croatia. Before (from the 11th century) the administrative seat of the former Modruš County (later renamed Modruš-Rijeka County), it is today just a ruin in the Josipdol Municipality in the southern part of the Karlovac County.