Mercer County is a county in Southern West Virginia on the southeastern border of the U.S. state of West Virginia. At the 2020 census, the population was 59,664. Its county seat is Princeton. The county was originally established in the State of Virginia by act of its General Assembly on March 17, 1837, using lands taken from Giles and Tazewell counties. Mercer County is part of the Bluefield, WV-VA Micropolitan Statistical Area.