Charleroi is a borough in Washington County, Pennsylvania, along the Monongahela River, 21 miles south of Pittsburgh. Charleroi was settled by Walloons 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, is known as the "Mon Valley"; or by some speakers, the school-league-term has long been extended to mean from the river mouth to northern West Virginia. Once dubbed "Magic City", Charleroi has in recent years seen a gradual revitalization of its business district.