Fairmont is a city in and the county seat of Marion County, West Virginia, United States. The population was 18,313 at the 2020 census, making it the eighth-most populous city in the state. It is the principal city of the Fairmont micropolitan area, which includes all of Marion County in North Central West Virginia and had a population of 56,205 in 2020. Fairmont is also a principal city of the larger Morgantown–Fairmont combined statistical area.