The Catskill Mountains, also known as the Catskills, are a physiographic province and subrange of the larger Appalachian Mountains, located in southeastern New York. 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 700,000-acre (2,800 km²) forest preserve protected from many forms of development under New York state law.

Geologically, the Catskills are a mature dissected plateau, a flat region subsequently uplifted and eroded into sharp relief by watercourses. The Catskills form the northeastern end of the Allegheny Plateau (also known as the Appalachian Plateau).

The Catskills were named by early Dutch settlers. They are well known in American society as the setting for films and works of art, including many 19th-century Hudson River School paintings, as well as for being a favored destination for vacationers from New York City in the mid-20th century. The region's many large resorts gave many young stand-up comedians an opportunity to hone their craft. Since the late 19th century, the Catskills have been a haven for artists, musicians and writers, especially in and around the towns of Woodstock and Phoenicia.

Nicolaes Visscher I's 1656 map of New Netherland located the Landt van Kats Kill at the mouth of Catskill Creek. The region to the south is identified as Hooge Landt van Esopus (High Lands of the Esopus), a reference to a local band of northern Lenape Native Americans who inhabited the banks of the Hudson and hunted in the highlands along the Esopus Creek.

While the meaning of the name ("cat creek [kill]" in Dutch) and the namer (early Dutch explorers) are settled matters, how and why the area is named "Catskills" is a mystery. Mountain lions (catamounts) were known to have been in the area when the Dutch arrived in the 17th century and may have been a reason for the name.

The confusion over the origins of the name led over the years to variant spellings such as Kaatskill and Kaaterskill, both of which are also still used: the former in the regional magazine Kaatskill Life, the latter as the name of a mountain peak and a waterfall. Henry Schoolcraft is supposed to have coined the indigenous term for the range, On-ti-o-ra ("mountains/hills/land of the sky"), in 1844, due to the term not being attested prior to his record of it.

The Catskill Mountains are approximately 100 miles (160 km) north-northwest of New York City and 40 miles (64 km) southwest of Albany, starting west of the Hudson River. The Catskills occupy much of two counties (Greene and Ulster), and extend slightly into Delaware, Sullivan, and southwestern Schoharie counties.

At the range's eastern end, the mountains begin dramatically with the Catskill Escarpment rising up suddenly from the Hudson Valley. The western boundary is far less certain, as the mountains gradually decline in height and grade into the rest of the Allegheny Plateau, but maps from the 18th and 19th centuries consistently mark the border of the Catskill Mountains as the East Branch of the Delaware River, which is consistent with the actual topographic relief. The Pocono Mountains, to the immediate southwest in Pennsylvania, are also a part of the Allegheny Plateau.