Muddy is a small incorporated village located in the Harrisburg Township in Saline County, Illinois, United States. The population was 68 at the 2010 census. Muddy was built as a coal mining village to house miners working in O'gara #12 mine located on the north bank of the Saline River. Its sister city of Harrisburg is located on the south bank.
Muddy, until 2002, held the smallest post office in the United States. A larger building adjacent to the former post office was constructed to better serve its citizens. The old post office remains, and the sign now reads "OLD MUDDY POST OFFICE".