Bethlehem Township is a township in Hunterdon County, New Jersey, United States. 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.
Bethlehem was first mentioned in official records dating back to 1730, though details of its formation are uncertain. Bethlehem was incorporated as one of New Jersey's initial 104 townships by an Act of the New Jersey Legislature on February 21, 1798. Portions of the township were taken to form Kingwood Township (1749, date uncertain), Alexandria Township (March 5, 1765), Union Township (February 17, 1853), Junction borough (February 20, 1895, now known as Hampton borough), Bloomsbury (March 30, 1905) and Glen Gardner (March 26, 1919).
The Musconetcong River forms the township's northern border with Warren County. The northern half of the Township consists of the Musconetcong Valley while the southern half is covered by the Musconetcong Mountains. The southwest corner of the township lies on what is known as the Hunterdon Plateau.