Rocky Brook is a tributary of the Millstone River in Monmouth and Mercer counties, New Jersey, in the United States.

It flows parallel to CR-1 (Sweetmans Lane) before draining into Perrineville Lake.

It continues flowing west into the northern areas of Perrineville Lake Park.

[2] It continues flowing west alongside Etra Road and crosses the New Jersey Turnpike.

It then flows alongside the East Windsor Open Space Acquisition before draining into the Millstone River at 40°17′18″N 74°32′23″W / 40.28833°N 74.53972°W / 40.28833; -74.53972.