Seongdeok Daewang (; reigned 702âÂÂ737) was the thirty-third king of the ancient Korean kingdom of Silla. He was the second son of King Sinmun and the younger brother of King Hyoso.
In 704 Seongdeok married Lady Baeso (posthumous Queen Seongjeong), a daughter of Kim Wontae; their son Junggyeong was named Crown Prince in 715. In 716 Seongjeong was dismissed from the palace, and Junggyeong died the following year. In 720 Seongdeok married Queen Sodeok, daughter of the minister Kim Sun-won; their sons included future kings Hyoseong and Gyeongdeok.
SillaâÂÂTang relations improved markedly in SeongdeokâÂÂs time after decades of confrontation. In 733, responding to Tang requests to check Balhae, Silla mobilized forces for a joint campaign; severe winter conditions forced a withdrawal en route. In 735, the Tang court formally recognized SillaâÂÂs control of the territory south of the Pae (Taedong) River, consolidating SillaâÂÂs northern boundary.
In 721 Seongdeok ordered the construction of a long defensive wall along SillaâÂÂs northern frontier, interpreted in scholarship as a response to BalhaeâÂÂs southward expansion along the East Sea coast. In 722 a large mountain fortress was built near the capital Gyeongju, known as Gwanmunseong (also Mobeol-gun Fortress); medieval sources report the mobilization of nearly 40,000 workers and a circumference of roughly 12 km.
In 718 the court established the Nugakjeon (æ¼Âå»堸), an Office of Timekeeping that managed state water clocks; the Samguk sagi records the first making of a water clock (nugak, æ¼Âå») in the same year. Land policy also evolved: in 722 the state is reported to have first distributed jeongjeon (ä¸Âç°, âÂÂable-bodied fieldsâÂÂ) to commoners of working age; although details remain debated, the measure is commonly interpreted as an attempt to stabilize peasant tenure and strengthen royal authority.
Chinese annals record that in the second lunar month of 737 a Tang envoy was dispatched to invest SeongdeokâÂÂs successor (later King Hyoseong), leading some scholars to infer that Seongdeok may have died late in 736.