Manalapan Brook is a tributary of the South River in Monmouth and Middlesex counties, New Jersey in the United States.

[2] The Manalapan Brook's source is at 40°11′26″N 74°23′51″W / 40.19056°N 74.39750°W / 40.19056; -74.39750[3] in western Monmouth County, a few miles south of Monmouth Battlefield State Park, and roughly adjacent to the headwaters of the Manasquan River, Matchaponix Brook and Millstone River systems in the much larger Raritan Basin.

[1][2][4][5] This long brook flows generally north, traversing, along with its tributaries, much of Monmouth and Middlesex Counties.

The flow then merges with the Matchaponix Brook forming the South River at 40°23′33″N 74°22′58″W / 40.39250°N 74.38278°W / 40.39250; -74.38278[3] along the border of Spotswood and Old Bridge Township.

Manalapan in the Unami Lenape dialect more correctly corresponds to Man- "tilt/slant" Alapan- "Sunny weather"- combined would describe sun rays through Manalapan's forests and fields, which were maintained by the Lenape by a combination of localized syntropic farming and hunter-gathering lifestyle.