Buchanan County is a United States county in far western Virginia, the only county in the state to border both West Virginia and Kentucky. The county is part of the Southwest Virginia region and lies in the rugged Appalachian Plateau portion of the Appalachian Mountains. Its county seat is Grundy. Buchanan County was established in 1858 from parts of Russell and Tazewell counties, and it was named in honor of then-President James Buchanan.