Charleroi (/ˈʃɑːrlərɔɪ/ SHAR-lə-roy) is a borough in Washington County, Pennsylvania, along the Monongahela River, 21 miles south of Pittsburgh. Charleroi was settled in 1890 and incorporated in 1891. The 2010 census recorded a population of 4,120.
There has been a large-scale cessation of industrial activities in the region. The decrease in the population is associated with the decline of regional heavy and medium industries, especially the steel-making industry, all once fed by the cheap transportation on the Monongahela River which extends from upstream of Charleroi well into northern West Virginia and north & downstream past McKeesport to the mouth of the Monongahela at Pittsburgh. Colloquially, the stretch from Charleroi north to McKeesport (historically because of press coverage of High School sports leagues), is known as the "Mon Valley"; or by some speakers (politicians, reporters and weathermen), the school-league-term has long been extended to mean from the river mouth to northern West Virginia.