Charleroi is a borough in Washington County, Pennsylvania, along the Monongahela River, 25 miles south of Pittsburgh. Charleroi was settled in 1890 and incorporated in 1891. The population in 1900 stood at 5,930; in 1910, 9,615; in 1920, 11,516, and in 1940, 10,784. The population was 4,871 at the 2000 census. There has been a large-scale cessation of industrial activities in the region. The decrease in the population is associated with the decline of the steel-making industry in the "Mon Valley" which extends from Charleroi to the mouth of the Monongahela at Pittsburgh.