Green Valley is an unincorporated area and census-designated place (CDP) in southern Frederick County, Maryland, United States.

The northern edge of the CDP extends from Mount Airy in the northeast to New Market in the northwest.

According to the United States Census Bureau, the Green Valley CDP had a total area of 20.6 square miles (53 km2), all land.

There were 3,928 housing units at an average density of 190.6 per square mile (73.6/km2).

7.8% of all households were made up of individuals, and 2.8% had someone living alone who was 65 years of age or older.