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King Sejo of Joseon (hangul: 세조; hanja: 世祖; 1417-1468, r. 1455-1468) was the seventh king of the Joseon Dynasty of Korea. Born in 1417 as Yi Yu, King Sejong the Great's second son, he showed great ability at archery, horseriding and martial arts. He was also a brilliant military commander, though he never went to the battlefront himself. He became Grand Prince Suyang in 1428, the name by which he was better known.

Following King Sejong's death, Suyang's ill brother, Munjong, took the throne but soon died. The crown passed to his 12-year-old son, Danjong. The new king was too young to rule the nation, and all political processes were controlled by then-premier Hwangbo In and General Kim Jongseo, who was vice-premier. As Kim Jongseo and his faction used the chance to extend the power of court officials against many royal family members, the tension between Kim and Suyang greatly increased; not only Suyang himself, but his younger brother -Grand Prince Anpyong- also sought an opportunity to take control of the kingdom.

Suyang surrounded himself with trusted allies, including his famous adviser Han Myung-Hoi. Han advised Suyang to take over the government in a coup, and in October 1453, he killed Kim Jongseo and his faction, thereby taking the reins of power into his own hands. After the coup, he arrested his brother Anpyong, first sending him into exile, then putting him to death. Finally, in 1455, he forced his powerless young nephew Danjong to abdicate, declaring himself seventh king of the Joseon dynasty. Later he demoted Danjong to prince and ordered him to be poisoned after his younger brother, Grand Prince Geumsung, and later six scholars plotted to remove the Suyang from power inan attempt to put Danjong back on the throne.

Despite having snatched the throne from his young nephew, killing many people in the process, he proved himself one of the most able rulers and administrators in Korean history. First, he strengthened the monarchy established by Taejong, by weakening the power of the prime minister and bringing staff directly under the king's control. He also strengthened the administrative system, which had also been introduced by Taejong, enabling the government to determine exact population numbers and to mobilize troops effectively. Just like Taejong, he was a hardliner with regards to foreign policy, attacking Jurchens on the northern front in 1460 (오랑캐/兀良哈) and 1467 (호리개/胡里改). He also revised the land ordinance to improve the national economy. He executed scholars from King Sejong's era for plotting against him, but encouraged publishing, including several history, economics, agricultural, and religious books. Most importantly, he compiled the Grand Code for State Administration, which became the cornerstone of dynastic administration and provided the first form of constitutional law in a written form in Korea. He died in 1468, and the throne passed to his weak son, Yejong.

• List of Korean monarchs#Joseon