The Adirondacks are a group of mountains in north-eastern New York, U.S.A., in Clinton, Essex, Franklin and Hamilton counties, often included by geographers in the Appalachian system, but pertaining geologically to the Laurentian highlands of Canada. They are bordered on the east by Lake Champlain, which separates them from the Green Mountains.

A large portion of the Adirondack range is encompassed within the constitutionally protected 6 million acres of Adirondack State Park, which includes a forest preserve of approximately 2.3 million acres. The Adirondacks contain a number of lakes, including Lake Placid, two-time site of the Olympic Winter Games.

Unlike the Appalachians, the Adirondacks do not form a connected range, but consist of many summits, isolated or in groups, arranged with little appearance of system. There are about one hundred peaks, ranging from 370 m to 1500 m (1200 to 5000 ft.) in height; the highest peak, Mt. Marcy (called by the Indians Tahawus or "cloud-splitter"), is near the eastern part of the group and attains an elevation of 1629 m (5344 ft). Other noted peaks are McIntyre, 1588 m (5210 ft), Haystack 1499 m (4918 ft.), Dix 1498 m (4916 ft.) and Whiteface 1485 m (4871 ft).

These mountains, consisting of various sorts of gneiss, intrusive granite and gabbro, have been formed partly by faulting but mainly by erosion, the lines of which have been determined by the presence of faults or the presence of relatively soft rocks. Lower Palaeozoic strata lap up on to the crystalline rocks on all sides of the mountain group. The region is rich in magnetic iron ores, which was mined for many years. Other mineral products are graphite, garnet used as an abrasive, pyrite and zinc ore.

The mountains form the water-parting between the Hudson and the St. Lawrence rivers. On the south and south-west the waters flow either directly into the Hudson, which rises in the centre of the group, or else reach it through the Mohawk. On the north and east the waters reach the St. Lawrence by way of Lakes George and Champlain, and on the west they flow directly into that stream or reach it through Lake Ontario. The most important streams within the area are the Hudson, Black, Oswegatchie, Grass, Raquette, Saranac and Ausable rivers.

The region was once covered, with the exception of the higher summits, by the Laurentian glacier, whose erosion, while perhaps having little effect on the larger features of the country, has greatly modified it in detail, producing lakes and ponds, whose number is said to exceed 1300, and causing many falls and rapids in the streams. Among the larger lakes are the Upper and Lower Saranac, Big and Little Tupper, Schroon, Placid, Long, Raquette and Blue Mountain. The region known as the Adirondack Wilderness, or the Great North Woods, embraces between 13,000 km2 and 16,000 km2 (5000 and 6000 square miles) of mountain, lake, plateau and forest, which for scenic grandeur is almost unequalled in any other part of the United States.