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.

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.

The 2010 census reported racial makeup was 88.7% White, 0.4% American Indian and Alaska Native, and 10.9% other races.