Westfield is a town in Union County, New Jersey, United States. As of the 2010 United States Census, the town population was 30,316. The old village area, now the downtown district, was settled in 1720 as part of the Elizabethtown Tract.

Westfield was originally formed as a township on January 27, 1794, from portions of Elizabeth Township, while the area was still part of Essex County, and was incorporated as a one of New Jersey's first 104 townships by an Act of the New Jersey Legislature on February 21, 1798. It became part of the newly formed Union County on March 19, 1857. Portions of the township have been taken to form Rahway Township (February 27, 1804), Plainfield Township (April 5, 1847), Cranford Township (March 14, 1871), Fanwood Township (March 6, 1878, now known as Scotch Plains), Mountainside (September 25, 1895) and Hillside (April 3, 1913). The Town of Westfield was incorporated on March 4, 1903, replacing Westfield Township.

Westfield is located at 40°39′06″N 74°20′36″W / 40.651644°N 74.343447°W / 40.651644; -74.343447 (40.651644,-74.343447). According to the United States Census Bureau, the town had a total area of 6.743 square miles (17.463 km²), of which, 6.719 square miles (17.401 km²) of it is land and 0.024 square miles (0.062 km²) of it (0.36%) is water.