St. Charles Airport (FAA LID: 3SQ) was a privately owned, public use airport located four nautical miles (7.41 km) north of the central business district of the City of St. Charles in St. Charles County, Missouri, United States.^[1] According to the FAA's National Plan of Integrated Airport Systems for 2007–2011, it was categorized as a reliever airport.^[2] The airport closed on June 30, 2010, and was restored to agricultural use farmland.

Although most U.S. airports use the same three-letter location identifier for the FAA and IATA, St. Charles Airport was assigned 3SQ by the FAA but had no designation from the IATA.^[3]

St. Charles Airport covered an area of 72 acres (29 ha) at an elevation of 442 feet (135 m) above mean sea level. It had one asphalt paved runway designated 09/27 which measured 3,451 by 50 feet (1,052 x 15 m) and two turf runways: 15/33 is 2,310 by 103 feet (704 x 31 m) and 18/36 is 2,145 by 100 feet (654 x 30 m).^[1]

For the 12-month period ending August 27, 2007, the airport had 40,260 aircraft operations, an average of 151 per day: 95% general aviation and 5% air taxi. At that time there were 77 aircraft based at this airport: 88% single-engine, 10% multi-engine, and 1% ultralight.^[1] St. Charles Flying Service was based at the field and moved to the nearby St. Charles Smartt airport.