Hengdong County () is a county located in the east of Hengyang prefecture-level city, Hunan province, China. It was established from the eastern part of Hengshan County in 1966 and named after the east of Mount Heng. The county has an area of with a population of 746,300 (as of 2015). The county of Hengdong has two townships and 15 towns under its jurisdiction, its seat is Mishui Town ().