Catskill Mountains, or Catskills, low, rounded mountains in southeastern New York, just west of the Hudson River. They cover an area of about 1,400 square miles (3,600 km2), mainly in Green, Delaware, and Ulster counties. The Catskills make up part of the Appalachian system and, geologically, are the dissected remains of the old plateau. They were heavily scoured by glaciers during the last Ice Age.

Most of the peaks in the Catskills reach elevations of roughly 3,000 feet (900 m) above sea level. The highest is Slide Mountain, which rises to 4,204 feet (1,281 m). The Catskills have extensive forests, deep valleys and gorges, and numerous waterfalls. Their nearness to the New York City metropolitan area has helped make them a popular summer and winter resort area. Man-made reservoirs, including Ashokan, Pepacton, and Rondout, supply water to New York City. Much of the Catskill region is included in Catskill Park, a New York state forest preserve and scenic and recreational park.