Sejo (; 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 Councilor Hwangbo 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 () and 1467 (). 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 () and Yeokdaebyeongyo (). 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 () and is located in Namyangju, South Korea.

Worship In Korean shamanism, Sejo is worshiped as a wangshin ().

Family Father: King Sejong (; 15 May 1397 – 8 April 1450) Grandfather: King Taejong (; 21 June 1367 – 8 June 1422) Grandmother: Queen Wongyeong (), of the Yeoheung Min clan (; 6 August 1365 – 27 August 1420) Mother: Queen Soheon (), of the Cheongsong Shim clan (; 20 October 1395 – 28 April 1446) Grandfather: Shim On (), Internal Prince Cheongcheon (; 1375 – January 1419) Grandmother: Grand Princess Consort of Samhan State (), of the Sunheung An clan (; ?–1444) Consort(s) and their respective issue Queen Jeonghui (), of the Papyeong Yun clan (; 25 December 1418 – 15 May 1483) Yi Jang (), Crown Prince Uigyeong (; 12 October 1438 – 29 September 1457), first son Princess Uisuk (; 1441 – 15 January 1478), first daughter Princess Uiryeong (), personal name Se-hui (), disputed daughter Yi Hwang (), King Yejong (; 23 January 1450 – 9 January 1470), third son Concubine Geun (), of the Seonsan Park clan (; 1425–?) Yi Seo (), Prince Deokwon (; 7 April 1449 – 18 August 1498), second son Yi Seong (), Prince Changwon (; 1458 – 14 September 1484), fourth son Soyong (), of the Park clan (; ? – 3 October 1465) Unnamed son (1459–1463) Sugwon (), of the Goryeong Shin clan ()

Ancestry

In popular culture Sejo was a prominent character in The Ume Tree in the Midst of the Snow (), 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.

Portrayed by Kim Al-eum in the 1983 MBC TV series The King of Chudong Palace. Portrayed by Nam Sung-woo in the 1984–1985 MBC TV series 500 Years of Joseon: The Ume Tree in the Midst of the Snow. Portrayed by Seo In-seok in the 1994 KBS2 TV series Han Myung-hoi. Portrayed by Im Dong-jin in the 1998–2000 KBS1 TV series The King and the Queen. Portrayed by Choi Bong-sik in the 2007 KBS2 TV series Sayuksin. Portrayed by Kim Byung-se in the 2007–2008 SBS TV series The King and I. Portrayed by Kim Yeong-cheol in the 2011 KBS2 TV series The Princess' Man. Portrayed by Kim Young-ho in the 2011 JTBC TV series Insu, the Queen Mother. Portrayed by Lee Jung-jae in the 2013 film The Face Reader. Portrayed by Go Young-bin in the 2016 KBS1 TV series Jang Yeong-sil. Portrayed by Park Hee-soon in the 2019 film Jesters: The Game Changers. Portrayed by Kim Nam-gil in the 2026 film Canvas of Blood.

See also History of Korea List of monarchs of Korea Styles and titles in Joseon Korean–Jurchen border conflicts

Notes

References