Belmont County is a county in the U.S. state of Ohio. As of the 2020 United States census, the population was 66,497. Its county seat is St. Clairsville, while its largest city is Martins Ferry. The county was created on September 7, 1801, and organized on November 7, 1801. It takes its name from the French for "beautiful mountain".

St. Clairsville or Saint Clairsville is a city in and the county seat of Belmont County, Ohio, United States. The population was 5,096 at the 2020 census. It is part of the Wheeling metropolitan area.