Middlebush Brook is a tributary of Six Mile Run in Somerset County, New Jersey in the United States.

One closely parallels South Middlebush Road before splitting into two more tributaries.

It exits the commerce area and flows through a marsh, then crossing Bennetts Lane.

It crosses Blackwells Mills Road a little later on, then drains into Six Mile Run at 40°28′21″N 74°33′02″W / 40.47250°N 74.55056°W / 40.47250; -74.55056.

This stream has a streambed composed of small boulders, making it difficult to walk on.