Bethlehem Township is a township in Hunterdon County, New Jersey, United States. It is part of the New York metropolitan area. As of the 2010 United States Census, the township's population was 3,979, reflecting an increase of 159 (+4.2%) from the 3,820 counted in the 2000 Census, which had in turn increased by 716 (+23.1%) from the 3,104 counted in the 1990 Census.