Union Township is a township in Hunterdon County, New Jersey, United States. As of the 2010 United States Census, the township's population was 5,908, reflecting a decline of 252 (-4.1%) from the 6,160 counted in the 2000 Census, which had in turn increased by 1,082 (+21.3%) from the 5,078 counted in the 1990 Census. The southwest half of the township lies on what is known as the Hunterdon Plateau, the northwest corner consists of the Musconetcong Ridge and the northeast section is part of the lower-lying Newark Basin around Spruce Run Reservoir.
Union was incorporated as a township by an Act of the New Jersey Legislature on February 17, 1853, from portions of Bethlehem Township. Clinton Town was formed on April 5, 1865, within portions of the township, and became an independent municipality in 1895.
The township was named for Union Furnace, which was producing iron as early as 1700. Union Furnace and its forge produced cannon balls for the Revolutionary War and shoes for horses and oxen, as well as farm implements. Forests gave way to farm fields as trees were cut down to stoke the furnace. A farm community then developed along with the accompanying industries of basket making and tanning.
New Jersey Monthly magazine ranked Union Township as its 21st best place to live in its 2008 rankings of the "Best Places To Live" in New Jersey.