Massac County is a county in the U.S. state of Illinois. According to the 2020 census, it had a population of 14,169. Established in 1843 and named for a French fort founded in the 18th century, its county seat is Metropolis. Massac County is included in the Paducah, KY-IL Metropolitan Statistical Area. It is located along the Ohio River, in the portion of the state known locally as "Little Egypt".