Sejo (Korean:세조;Hanja:世祖; 11 November 1417 – 2 October 1468), personal name Yi Yu, sometimes known as Grand Prince Suyang, was the seventh monarch of Joseon. He was the second son of Sejong the Great and the uncle of King Danjong, whom he forced to abdicate in 1455, after having previously led a coup d'état against him two years earlier. In his quest to seize power he ruthlessly killed rivals, including his younger brothers, Grand Prince Anpyeong and Grand Prince Geumseong. However, during his reign, he reformed administration, led campaigns against the Jurchens, revised land laws, promoted literature, and established court music. His Kyŏngguk Taejŏn (State Code) became Joseon's foundational legal framework.
Biography
Early life
Born on 11 November 1417 as the fourth child and second son of Grand Prince Chungnyeong (future King Sejong) by his primary wife, Grand Princess Consort of Samhan State (future Queen Soheon), he showed great ability at archery, horse riding and martial arts, and was also a brilliant military commander, though he never went to the battlefront himself. He also possessed musical talent, which delighted his father, who claimed that the prince could achieve many great things if he put his mind to it.
On 6 June 1428, during King Sejong’s 10th year of reign, he received the title Grand Prince Jinpyeong. That same year on 13 October, he married Lady Yun (the future Queen Jeonghui), the youngest daughter of Yun Beon who was Deputy Director of Military Armaments.
On 27 June 1433, his title changed to Grand Prince Hampyeong, but Grand Prince Hamheung was alternately used as well. On 1 July, his title changed again to Grand Prince Jinyang as there were concerns of the previous title being confused with Hampyeong-hyeon in Jeolla Province.
In 1445, his title was lastly changed to Grand Prince Suyang, by which he is commonly known as.
Rise to power
Following the death of King Sejong in 1450, Suyang's ill brother, Yi Hyang (later known as King Munjong), took the throne but died two years later, and the crown passed to his 12-year-old son, Yi Hongwi (posthumously named King Danjong). The new monarch was too young to rule the nation, and the government was controlled by Chief State CouncilorHwangbo In and General Kim Chongsŏ, who was the Left State Councilor. As Kim Chongsŏ and his faction used the chance to extend the power of court officials against royal family members, the tension between him and Suyang greatly increased. Both Suyang and his younger brother, Grand Prince Anpyeong, sought opportunities to take control of the country.
In order to court the support of the Ming dynasty, Suyang became an ambassador in 1452. He also surrounded himself with trusted allies, including his famous tactician, Han Myŏnghoe, who advised him to take over the government in a coup. In 1453, Suyang killed Kim Chongsŏ and his faction, thereby taking the reins of power into his own hands. After the coup, he arrested his younger brother, Grand Prince Anpyeong, first sending him into exile, then sentencing him to death.
Reign
Finally, in 1455 Suyang forced the powerless king to abdicate, declaring himself the new ruler of Joseon (today known by the temple name "Sejo").
After his younger brother Grand Prince Geumsung, and six scholars, including Sŏng Sammun, Pak P'aengnyŏn and Yi Kae, plotted to remove him from power in an attempt to put his nephew back on the throne, Suyang demoted the former king Danjong from "King Emeritus" (Sangwang, 상왕, 上王) to "Prince Nosan" (Nosan-gun, 노산군, 魯山君) and later ordered him to commit suicide by poison.
Despite having snatched the throne from his young nephew and killing many people in the process, Sejo proved himself one of the ablest rulers and administrators in Korean history. First, he continued King Taejong's legacy of strengthening the monarchy by weakening the power of the State Council and bringing the officials directly under the king's control. He also further developed the administrative system, which had also been introduced by Taejong, enabling the government to determine exact population numbers and to mobilize troops effectively (this caused Yi Si-ae's Rebellion, which he suppressed). Just like Taejong, Sejo was a hardliner concerning foreign policy and attacked the 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 and encouraged the publication of history, economy, agriculture, and religion books.
Sejo himself compiled a number of books based on his interests. One of them is Seokbosangjeol, a biography of Gautama Buddha. The others are Worinseokbo (월인석보;月印釋譜) and Yeokdaebyeongyo (역대병요;歷代兵要). One of his crowning achievements was the cultural progress Joseon made during his reign, such as the establishment of Jongmyo court music. His father, King Sejong, had always wanted to use Korean music rather than Chinese music for ancestral ritual, but conservative court officials thought that Chinese music was far superior to Korean music and stopped Sejong's efforts. However, when King Sejo rose to the throne, he modified the ritual music composed by his father and used it for royal ancestral rituals, which is now inscribed as a UNESCO Intangible Cultural Heritage for Humanity.
Most importantly, he compiled the State Code, which became the cornerstone of dynastic administration and provided the first form of written constitutional law in Korea.
Death
Sejo died in 1468, and the throne passed to his sickly third son, Yi Hwang (King Yejong). His tomb is known as Gwangneung (광릉) and is located in Namyangju, South Korea.
Worship
In Korean shamanism, Sejo is worshiped as a wangshin (왕신;王神;lit.''king god'').
Family
Father: King Sejong (세종대왕; 15 May 1397 – 8 April 1450)
Grandfather: King Taejong (태종대왕; 21 June 1367 – 8 June 1422)
Queen Soheon, of Cheongsong Shim clan (소헌왕후 청송 심씨)
An Cheon-bo (안천보)
Grand Princess Consort of Samhan State, of the Sunheung An clan (삼한국대부인 순흥 안씨)
Lady, of the Yeongju Kim clan (영주 김씨)
In popular culture
Sejo was a prominent character in The Ume Tree in the Midst of the Snow (설중매), the third installment of MBC's monumental series 500 Years of Joseon. Moon Ki-hoon of The Korean Herald labelled the depiction "a creative betrayal of the historical record", arguing that it attempted to rehabilitate Sejo's status as an usurper. Moon linked the depiction to political coercion from president Chun Doo-hwan, who gained his position from a coup in 1979.
The 2026 film The King's Warden depicts Danjong's exile and death. Though Sejo is not seen onscreen, the popularity of the film led to Sejo's royal tomb being review-bombed on several navigation apps.
↑Known as honorific name (존호; 尊號) in the Sinosphere, its closest equivalent in the English-speaking world is the regnal name.
↑From 1445 to 1455. Previously, Grand Prince Jinpyeong (진평대군; 晉平大君) from 1428 to 1433; Grand Prince Hampyeong (함평대군; 咸平大君) in 1433; and Grand Prince Jinyang (진양대군; 晉陽大君) from 1433 to 1445.
↑In the Korean calendar (lunisolar), he was born on the 24th day of the 9th lunar month and died on the 8th day of the 9th lunar month.
↑She is only mentioned in an unofficial history called Kŭmgye P'iltam (금계필담) written in 1873 by Seo Yu-yeong (서유영). The sole reference at her existence in an official document is a passage from the Veritable Records of the Joseon Dynasty, where Grand Prince Suyang (as Sejo was still known at the time) is recorded as having "one son and two daughters" [Sejong Sillok, year 28].
↑Demoted to palace maid (내인), then executed by hanging following a second offense.
# denotes that the king was deposed and never received a temple name.
↑Those who were listed were not reigning monarchs but posthumously recognized; the year following means the year of recognition.
↑Only the crown princes that did not become the king were listed; the former year indicates when one officially became the heir and the latter one is that when one died/deposed. Those who ascended to the throne were excluded in the list for simplification.
↑The title given to the biological father, who never reigned, of the kings who were adopted as the heir to a precedent king.
↑The de jure monarch of Korea during the era was the Emperor of Japan, while the former Korean emperors were given nobility title "King Yi" instead.
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