Watchung Reservation is the largest nature reserve in Union County, New Jersey, United States. The reservation consists mainly of the upper valley of Blue Brook, between the ridges of the First and Second Watchung Mountains. A dam near the headwaters of the creek creates Lake Surprise.

The reservation covers an area of 1,945 acres (787 ha). Parts of the reservation lie within the city of Summit, the borough of Mountainside, and the townships of Berkeley Heights, Scotch Plains, and Springfield.

The network of trails criss-crossing the reservation are popular with hikers and horseback riders. Biking on the trails is illegal.

The Blue Brook has carved a valley in the reservation between the 1st and 2nd Watchung Mountains along the strike of the less weathering-resistant red beds of the early Jurassic Feltville Formation.

The Feltville Formation is a mapped bedrock unit primarily in New Jersey named for the Deserted Village of Feltville in the reservation, which is near where its type section was described.

Red sandstone and shale beds of this formation crop out along stream level and along an old, abandoned mill race that follows the north side of the stream. On the south side of the stream are exposures of a highly fractured flow of the base of the Orange Mountain Basalt.

The best exposures of the red beds are along the abandoned mill race and along cut banks in the stream and its tributaries.

Paleontologist Paul E. Olsen uncovered ancient dinosaur fossils in the reservation in the 1970s.