Plainfield is a census-designated place (CDP) in Cumberland County, Pennsylvania, United States. The population was 399 at the 2010 census. It is part of the Harrisburg–Carlisle metropolitan statistical area.

The Harrisburg–Carlisle metropolitan statistical area, officially the Harrisburg–Carlisle, PA Metropolitan Statistical Area, and also referred to as the Susquehanna Valley, is defined by the Office of Management and Budget as an area consisting of three counties in South Central Pennsylvania, anchored by the cities of Harrisburg and Carlisle.