Monessen is a city in Westmoreland County, Pennsylvania, United States. The population was 8,669 at the 2000 census. In 1940, 20,257 people lived there. In 1990 the population was 13,026. Steel-making was a prominent industry in Monessen, which was a Rust Belt borough in the "Mon Valley" of southwestern Pennsylvania that became a third-class city in 1921. It is near Charleroi and Donora, two similar places with similar histories.