Newcastle is an unincorporated community and census-designated place in southwestern Iron County, Utah, United States. It lies along State Route 56, 30 miles west of Cedar City. 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. Newcastle is a small farming community which has a dairy, orchard, greenhouses producing house plants and tomatoes, and several fields.