Monessen is a city in Westmoreland County, Pennsylvania, United States. The population was 7,720 at the 2010 census. In 1940, 20,257 people lived there. In 1990 the population was 13,026. Monessen is the most southwestern municipality of Westmoreland County. 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. Monessen is part of the Pittsburgh metropolitan area, the Rivers of Steel National Heritage Area, as well as the Laurel Highlands.
Monessen, named for the Monongahela River and the industrial German city of Essen, was created by land speculators fairly late in the history of the Mon Valley, after neighboring towns had already been settled. The East Side Land Company bought land from various farmers, laid out the streets, and then sold the lots to prospective residents and employers. James M. Schoonmaker, who had made his fortune in coke, owned a controlling interest in the land company.