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.

Princeton is a city in and the county seat of Mercer County, West Virginia, United States. The population was 5,872 at the 2020 census. It is part of the Bluefield micropolitan area. The city hosts the Princeton WhistlePigs baseball club of the Appalachian League.