Decatur is the largest city in Macon County, Illinois, United States, and its county seat. The city was founded in 1829 and is situated along the Sangamon River and Lake Decatur in Central Illinois. As of the 2020 census, it had a population of 70,522. It is the 17th-most populous city in Illinois and the "fastest shrinking city" in Illinois. Decatur has an economy based on industrial and agricultural commodity processing and production. The city is home to Millikin University and Richland Community College.