South New Castle is a borough in central Lawrence County, Pennsylvania, United States. The population was 649 at the 2020 census. It is primarily a residential community which lies south of the city of New Castle, and is part of the Pittsburgh metropolitan area. The borough will officially be annexed into a connecting township on January 1, 2026.

Lawrence County is a county in the Commonwealth of Pennsylvania. As of the 2020 census, the population was 86,070. The county seat and largest city is New Castle.