Union County Airport is a public use airport in Union County, Ohio, United States. It is owned by the Union County Airport Authority and located one nautical mile (2 km) southeast of the central business district of Marysville, Ohio. This airport is included in the National Plan of Integrated Airport Systems for 2011–2015, which categorized it as a general aviation facility.Although most U.S. airports use the same three-letter location identifier for the FAA and IATA, this airport is assigned MRT by the FAA, but has no designation from the IATA (which assigned MRT to Moroak, Northern Territory, Australia).Facilities and aircraftUnion County Airport covers an area of 54 acres (22 ha) at an elevation of 1,021 feet (311 m) above mean sea level. It has one runway designated 9/27 with an asphalt surface measuring 4,218 by 75 feet (1,286 x 23 m).