 SR 9 is the main east–west highway of northwestern Loudoun County, connecting Leesburg with Hillsboro and the West Virginia cities of Charles Town and Martinsburg.As a result, the state highway and its West Virginia continuation are a major, overburdened commuter route between the Eastern Panhandle of West Virginia and Washington, D.C..

SR 9 begins at the West Virginia state line at Keyes Gap, a wind gap on top of Blue Ridge Mountain.The highway continues west as WV 9 to Charles Town.