Cairo is a city in Alexander County, Illinois in the United States. The population was 3,632 at the 2000 census. It is the county seat of Alexander County. Cairo is located at the confluence of the Mississippi and Ohio Rivers, and is the southernmost town in the state of Illinois. The rivers converge at what is the southernmost point in Illinois at Fort Defiance State Park, an American Civil War fort that was commanded by General Ulysses S. Grant, making Cairo the only city in Illinois completely surrounded by levees. It is part of the Cape Girardeau–Jackson, MO-IL Metropolitan Statistical Area. Several blocks in the town comprise the Cairo Historic District, listed on the National Register of Historic Places (NRHP). The Old Customhouse is also on the NRHP.