Purgatory Brook is a 5.8-mile-long (9.3 km) long tributary of the Souhegan River in southern New Hampshire. It rises in a marshy area on the border of Lyndeborough and Mont Vernon, and flows south over three small waterfalls, intersecting the Souhegan in Milford. For much of its length, it constitutes the border between Lyndeborough and Mont Vernon.
The northernmost cascade, Upper Purgatory Falls, drew many visitors in the late 19th and early 20th century when Mont Vernon was home to a number of hotels that drew summer visitors from the heat of Boston. At one time the area had a dance hall and a bowling alley, as well as camping sites - none of which exist today. Also, there used to be railings, and evidence of these are still visible as metal poles. Townwide celebrations drew hundreds of people up through the Great Depression, according to Mont Vernon town histories.
Today the upper falls can only be reached on the Mont Vernon side by four-wheel drive vehicles, and on the Lyndeborough side by a short walk from a town-maintained road. The lowest waterfall can only be reached by a short hiking trail. The middle waterfall can be reached along a public trail, the length of the brook.