Fayette County is a county located in the U.S. state of Illinois. As of the 2020 census, the population was 21,488. Its county seat is Vandalia, the site of the Vandalia State House State Historic Site. Ramsey Lake State Recreation Area is located in the northwestern part of this county.

Vandalia is a city in and the county seat of Fayette County, Illinois, United States. At the 2020 Census, the population was 7,458. Vandalia is on the Kaskaskia River, and 60 miles (97 km) northeast of the St. Louis area. The city served as the state capital of Illinois from 1819 until 1839, when the seat of state government moved closer to the center of the state in Springfield. In the early 19th century, Vandalia was the western terminus of the National Road from the East Coast. Since 1933, the Vandalia State House State Historic Site has sought to preserve and interpret the State House building and grounds, originally constructed in 1836.