Middle Brook is a tributary of the Raritan River in Somerset County, New Jersey.

The stream can be referred to as Rha-weigh-weiros, a Native American name meaning "running from a deep hole", on early maps of the area.

[2] The Middle Brook's western and eastern branches drain the valley, now known as Washington Valley, between the first and second Watchung Mountain ridges in Bridgewater Township and the western part of Warren Township.

The total drainage area is 17.2 square miles (45 km2).

The smaller Eastern Branch is also dammed and then cascades over the Buttermilk Falls in Washington Valley Park.