Anna is the largest city and retail trade center in Union County, Illinois, United States. Located in Southern Illinois, its population was 4,303 at the 2020 United States Census, a decline from 5,135 in 2000. It is known for being tied to its close neighbor Jonesboro, together known as Anna-Jonesboro, and of which the main public high school for the two towns is named. Anna is known for the Choate Mental Health and Development Center, a state facility that opened in 1869.