New Castle Northwest is an unincorporated area and census-designated place (CDP) in Lawrence County, Pennsylvania, United States.

Pennsylvania Route 18 (Wilmington Road) runs through the CDP, leading south into New Castle and north 18 miles (29 km) to Hermitage.

According to the United States Census Bureau, the CDP has a total area of 0.89 square miles (2.3 km2), of which 1.7 acres (6,884 m2), or 0.30%, are water.

There were 731 housing units at an average density of 853.3 per square mile (329.5/km2).

35.9% of all households were made up of individuals, and 25.9% had someone living alone who was 65 years of age or older.