The Allegheny Mountains are a part of the Appalachian mountain range located in the eastern United States. They run from northeast to southwest through West Central Pennsylvania, and the western part of Maryland and eastern West Virginia. They begin at the Allegheny Front, which has an elevational change of up to two thousand feet. Absolute elevations reach well over four thousand feet in West Virginia. The highest point in West Virginia, Spruce Knob, is located in the Allegheny Mountains on the Allegheny Front.

To the west, the Allegheny Mountains grade down into the dissected Allegheny Plateau and Cumberland Plateau. To the east of the Allegheny Mountains (and east of the Allegheny Front) are the Ridge-and-valley Appalachians.

Much of the Monongahela National Forest in West Virginia is in the Allegheny Mountains. These mountains also include the Dolly Sods Wilderness, Laurel Fork Wilderness , and Cranberry Wilderness.

The bedrock of the Alleghenies is mostly sandstone and metamorphosed sandstone, quartzite, which is extremely resistant to weathering. Prominent beds of resistant conglomerate can be found in some areas, such as the Dolly Sods area. When it weathers, it leaves behind a pure white quartzite gravel. The rock of the Alleghenies were formed during the Alleghenian orogeny.

Because of intense freeze-thaw cycles in the higher Alleghenies, there is little native bedrock exposed in most areas. The ground surface usually rests on a massive jumble of sandstone rocks, with air space between them, that are gradually moving down-slope. The crest of the Allegheny Front is an exception, where high bluffs are often exposed, exposing an exceptional view.