Union County is a county located on the central southern border of the U.S. state of Arkansas. As of the 2020 census, the population was 39,054. 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, which said that they were petitioning "in the spirit of Union and Unity." The county is directly adjacent to the south to Union Parish in the state of Louisiana.