Heondeok (Korean: 헌덕; r. 809–826) was the 41st to rule the Korean kingdom of Silla. He was the younger brother of King Soseong, and served as regent during the reign of Aejang.

In 790, Heondeok traveled to Tang China where he distinguished himself and received a high position. He returned to Silla, becoming regent after the death of his brother. In 809, he slew the now-adult Aejang and took the throne for himself.

In 810, Heondeok repaired the country's irrigation facilities. He also sent his son Kim Hŏnjang to Tang with gold and silver Buddhist images to pray for the emperor's eternal peace.

Heondeok's later reign saw the rebellion of Kim Hŏnch'ang in 822, and that of Kim Hŏnch'ang's son in the following year. Both were suppressed. In 824, troubled by threats from the north, the king ordered a 300-ri-long wall built near the Taedong River, which was then the country's northern border.

He is believed to be buried in the Tomb of King Heondeok in Gyeongju, South Korea.