The Catskill Mountains are a large area in the southeastern portion of the U.S. state of New York. They are located approximately 100 miles north-northwest of New York City and forty miles southwest of Albany, starting just west of the Hudson River. The Catskills occupy much or all of five counties (Delaware, Greene, Schoharie, Sullivan, and Ulster). As a cultural and geographic region, the Catskills are generally defined as those areas close to or within the borders of the Catskill Park, a 700000 acre forest preserve protected from many forms of development under New York state law.

Geologically, the Catskills are a mature dissected plateau, a once-flat region subsequently uplifted and eroded into sharp relief by watercourses. The Catskills form the northeastern end of, and highest-elevation portion of, the Allegheny Plateau (also known as the Appalachian Plateau). Although the Catskills are sometimes compared with the Adirondack Mountains further north, the two mountain ranges are not geologically related, as the Adirondacks are a continuation of the Canadian Shield. Similarly, the Shawangunk Ridge, which forms the southeastern edge of the Catskills, is part of the geologically distinct Ridge-and-Valley province, and is a continuation of the same ridge known as Kittatinny Mountain in New Jersey and Blue Mountain in Pennsylvania.

Climatically, the Catskills lie within the Allegheny Highlands forests ecoregion.

The Catskills are well known in American culture, both as the setting for many 19th-century Hudson River School paintings and as the favored destination for urban vacationers from New York City in the mid-20th century. The region's many large resorts gave countless young stand-up comedians an opportunity to hone their craft. In addition, the Catskills have long been a haven for artists, musicians, and writers, especially in and around the towns of Woodstock and Phoenicia, New York.