Johnson County is a county in the U.S. state of Illinois. According to the 2020 census, it has a population of 13,308. Its county seat is Vienna. It is located in the southern portion of Illinois known locally as "Little Egypt".

Vienna is a city in and the county seat of Johnson County, Illinois, United States, and it is the site of two well-known state penitentiaries. The population of Vienna was 1,434 at the 2010 census.