Newcastle (also New Castle) is an unincorporated community and census-designated place (CDP) in southwestern Iron County, Utah, United States. It lies along State Route 56, 30 miles (48 km) west of Cedar City. Its elevation is 5,312 feet (1,619 m) above sea level. Newcastle has a post office with the ZIP code of 84756. The population was 247 at the 2010 census. Newcastle was founded in 1910 by citizens of the more isolated Pinto.
The predominant religion is the Church of Jesus Christ of Latter-day Saints, which church's members settled the area.
Newcastle is a small farming community which has a dairy, orchard, greenhouses producing house plants and tomatoes, and several fields. Crops produced include wheat, oats, alfalfa, corn and potatoes.