Plainfield is a census-designated place (CDP) in Cumberland County, Pennsylvania, United States.

Plainfield is located in the eastern part of West Pennsboro Township at 40°12′7″N 77°17′12″W / 40.20194°N 77.28667°W / 40.20194; -77.28667 (40.202023, -77.286621),[4] along Pennsylvania Route 641 (Main Street), which leads east 5 miles (8 km) to the center of Carlisle, the county seat, and west 6 miles (10 km) to Newville.

The Pennsylvania Turnpike (Interstate 76) passes along the northern edge of the CDP, but the nearest exit is 8 miles (13 km) to the east, on the other side of Carlisle.

According to the U.S. Census Bureau, Plainfield has a total area of 0.64 square miles (1.67 km2), all of which is land.

17.0% of all households were made up of individuals, and 8.5% had someone living alone who was 65 years of age or older.