Adirondack Mountains, mountains in northeastern New York. They lie just south of the Canadian border, between the St. Lawrence and Mohawk valleys and the lowlands containing Lakes Champlain and George. The mountains cover some 8,000 square miles (21,000 km2).

The Adirondacks are usually considered as part of the Appalachians, although geologically they are an extension of the vast Canadian Shield. They are among the oldest mountains in the United States and consequently have an aged and worn appearance. Slopes are long and gentle; virtually nowhere is the terrain rugged. The loftiest part lies just west of Lake Champlain where Mount Marcy, the highest peak in New York, rises 5,344 feet (1,629 m) above sea level. Elsewhere, especially in the south and the west, elevations are considerably lower.

Lakes abound in the Adirondacks, mostly small, clear ones created by glaciers during the last Ice Age. Among the many rivers that drain the region are the Hudson, Ausable, Raquette, Oswegatchie, and Black. The Adirondacks are densely forested, chiefly with spruces, firs, beeches, birches, and maples. Much of this wilderness land—some 3,900 square miles (10,000 km2)—is within the state-owned Adirondack Park. At Blue Mountain Lake, in the park, is the Adirondack Museum.

Because of their scenic beauty, the Adirondacks have become a popular resort and recreation area. They offer a wide variety of facilities and activities. The chief attraction in fall is the brilliantly colored foliage; in winter it is primarily skiing. Most prominent among the resort towns are Lake Placid, site of the 1932 and 1980 Winter Olympics; Saranac Lake; and Tupper Lake. There are no large cities in the Adirondacks.