Union County is a county located in the U.S. state of Arkansas. As of the 2010 census, the population was 41,639. 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 county is directly adjacent to Union Parish in the state of Louisiana.