Berkeley County is a county located in the Eastern Panhandle region of the U.S. state of West Virginia. As of 2007, the population is 104,169, making it the second-most populous county in West Virginia, behind Kanawha. Its county seat is Martinsburg.
The county lies adjacent to the Washington-Baltimore Metropolitan Area and is one of three counties in Hagerstown-Martinsburg, MD-WV Metropolitan Statistical Area.
Due to its proximity to Washington, D.C., Berkeley County is the fastest growing county in the State of West Virginia and among the fastest growing in the entire country.