Newcastle is a city in and the county seat of Weston County, Wyoming, United States. The population was 3,532 at the 2010 census.
Newcastle is located at 43°51′11″N 104°12′34″W (43.853183, -104.209343). It is located at the southwest edge of the Black Hills, at the intersections of U.S. Route 16 and U.S. Route 85.
According to the United States Census Bureau, the city has a total area of 2.55 square miles (6.60 km2), all of it land.
As of the census of 2010, there were 3,532 people, 1,439 households, and 868 families residing in the city. The population density was 1,385.1 inhabitants per square mile (534.8/km2). There were 1,663 housing units at an average density of 652.2 per square mile (251.8/km2). The racial makeup of the city was 94.6% White, 0.4% African American, 1.6% Native American, 0.3% Asian, 0.1% Pacific Islander, 1.0% from other races, and 2.0% from two or more races. Hispanic or Latino of any race were 3.4% of the population.
There were 1,439 households of which 27.2% had children under the age of 18 living with them, 47.0% were married couples living together, 8.9% had a female householder with no husband present, 4.4% had a male householder with no wife present, and 39.7% were non-families.