Sejong of Joseon (15 May 1397 – 8 April 1450), personal name Yi Do (), widely known as Sejong the Great (), was the fourth ruler of the Joseon dynasty of Korea, and a neographer. Initially titled Grand Prince Chungnyeong (), he was born as the third son of King Taejong and Queen Wongyeong. In 1418, he was designated as heir in place of his eldest brother, Crown Prince Yi Je. Today, King Sejong is regarded as one of the greatest leaders in Korean history.
Despite ascending to the throne after his father's voluntary abdication in 1418, Sejong was a mere figurehead; Taejong continued to hold the real power and govern the country up until his death in 1422. Sejong was the sole monarch for the next 28 years, although after 1439 he became increasingly ill, and starting from 1442, his eldest son, Crown Prince Yi Hyang (the future King Munjong), acted as regent.
Sejong reinforced Korean Confucian and Neo-Confucian policies, and enacted major legal amendments (공법, 貢法). He personally created and promulgated the Korean alphabet (today known as hangul), encouraged advancements in science and technology, and introduced measures to stimulate economic growth. He dispatched military campaigns to the north and instituted the Samin Jeongchaek ("Peasants Relocation Policy"; 사민정책, 徙民政策) to attract new settlers to the region. To the south, he helped subjugate Japanese pirates, during the Ōei Invasion.
Although the appellation "the Great" (대왕, 大王) was posthumously given to almost every monarch from the Goryeo and Joseon dynasties, this style is usually associated with Gwanggaeto and Sejong.
He was born on 10 April 1397, which was later adjusted to 15 May, after Korea's adoption of the Gregorian calendar in 1896. This date is his officially recognized birthday, and is celebrated along with National Teachers Day in South Korea.
Sejong was the son of King Taejong by his wife, Queen Wongyeong. When he was twelve, he became Grand Prince Chungnyeong (충녕대군). During childhood, he was favored by his father over his two older brothers.