Union County Airport (ICAO: KMRT, FAA LID: MRT) 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.