Hengnan County () is a county located in the south of Hengyang prefecture-level city, Hunan province, China. It was established in 1952 and named after the south of Mount Heng. The county has an area of with a population of 1,069,800 (as of 2015). The county of Hengnan has one township and 21 towns under its jurisdiction, its seat is the town of Yunji ().