Jangsu (394–491, r. 412–491) was the 20th monarch of Goguryeo, the northernmost of the Three Kingdoms of Korea. He was born in 394 as the eldest son of Gwanggaeto. He became the crown prince in 409, and upon his father's death in 412, became the ruler at the age of 18.
Jangsu reigned during the golden age of Goguryeo, when it was a powerful empire and one of the great powers in East Asia. He continued to build upon his father's territorial expansion through conquest, but was also known for his diplomatic abilities. Like his father, Gwanggaeto the Great, Jangsu also achieved a loose unification of the Three Kingdoms of Korea. In addition, Jangsu's long reign saw the perfecting of Goguryeo's political, economic and other institutional arrangements. He is also noted for building the Gwanggaeto Stele, dedicated to his father. Jangsu's posthumous name means "Long Life", based on his longstanding reign of 79 years until the age of 97, the longest reign in East Asian history.
During his reign, Jangsu changed the official name of Goguryeo (Koguryŏ) to the shortened Goryeo (Koryŏ), from which the name Korea originates.
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