Sayreville is a borough in Middlesex County, in the U.S. state of New Jersey. Sayreville is within the heart of the Raritan Valley region, located on the south banks of the Raritan River, and also located on the Raritan Bay. As of the 2020 United States census, the borough's population was 45,345, an increase of 2,641 from the 2010 census count of 42,704, which in turn reflected an increase of 2,327 from the 40,377 counted in the 2000 census.