Martinsburg is a city in and the county seat of Berkeley County, West Virginia, United States. The population was 18,773 at the 2020 census, making Martinsburg the largest city in the Eastern Panhandle of West Virginia and the sixth-most populous city in the state. It is a principal city of the Hagerstown–Martinsburg metropolitan area extending into Maryland, which had 293,844 residents in 2020.