Monmouth Junction is an unincorporated community and census designated place (CDP) in South Brunswick, in Middlesex County, in the U.S. state of New Jersey. As of the 2020 census, the CDP's population was 8,895,
Geography
Monmouth Junction is in southwestern Middlesex County, in the western part of South Brunswick. It is bordered to the northwest by Heathcote, to the northeast by Deans, and to the east by Dayton, all within South Brunswick as well. It is bordered to the southwest by Plainsboro Township. U.S. Route 1 forms a small part of the community's western border. New Brunswick, the Middlesex county seat, is to the northeast, and Trenton, the state capital, is to the southwest.
According to the United States Census Bureau, the Monmouth Junction CDP has an area of , nearly triple its area in 2010. Within the CDP, are land and , or 0.57%, are water. Land in the community drains southwest toward Heathcote Brook and Devils Brook, tributaries of the Millstone River and part of the Raritan River watershed.