Flemington is a borough in and the county seat of Hunterdon County, in the U.S. state of New Jersey. Most of the borough is located in the Amwell Valley, a low-lying area of the Newark Basin, and the Raritan Valley, the South Branch of the Raritan River, which flows through the center of Flemington. Northwestern portions of the borough sit on the Hunterdon Plateau. As of the 2020 United States census, the borough's population was 4,876, its highest decennial count ever and an increase of 295 (+6.4%) from the 2010 census count of 4,581, which in turn reflected an increase of 381 (+9.1%) from the 4,200 counted in the 2000 census.