Sayreville is a borough located on the Raritan River, near Raritan Bay in Middlesex County, New Jersey, United States. As of the 2010 United States Census, the borough's population was 42,704, reflecting an increase of 2,327 (+5.8%) from the 40,377 counted in the 2000 Census, which had in turn increased by 5,391 (+15.4%) from the 34,986 counted in the 1990 Census.