Union County is a county located in the U.S. state of Arkansas. As of 2000, the population was 45,629. The county seat is El Dorado. The county was formed on November 2, 1829, and named in recognition of the citizens' petition for a new county, stating that they were petitioning "in the spirit of Union and Unity".

The El Dorado Micropolitan Statistical Area includes all of Union County. In 1931, a monument was established at the Union County border with Union Parish, Louisiana, through the private efforts of former

Arkansas Governor George Washington Donaghey (1856-1937), who was born in Union Parish and grew up in the border area before he moved as a teenager to Conway, Arkansas. As governor, he oversaw the construction of the state capitol building in Little Rock and brought about the establishment of the state health unit and its agricultural colleges.