Monessen is a city in southwestern Westmoreland County, Pennsylvania, United States, located along the Monongahela River. The population was 6,876 at the 2020 census. Steel-making was a prominent industry in Monessen, and it became a third-class city in 1921. With restructuring of the steel industry in the later 20th century and loss of jobs, it is considered a Rust Belt borough in the "Mon Valley" of southwestern Pennsylvania.