Yeonsangun or Prince Yeonsan (Korean: 연산군; Hanja: 燕山君; 2 December 1476 – 30 November 1506), personal name Yi Yung, was the 10th monarch of Joseon. He was the eldest surviving son of King Seongjong and Deposed Queen Yun, whose execution has been cited as the primary catalyst for his descent into madness. Often considered the worst tyrant in Joseon's history and perhaps all Korean history, Yeonsangun is notorious for launching two bloody purges, seizing hundreds of women from all over the peninsula to serve as palace entertainers, and appropriating Sungkyunkwan as a personal pleasure ground. His despotic rule provided a stark contrast to the liberal era of his father, and as a much-despised overthrown king, he did not receive a temple name.

Lady Yun, later known as the Deposed Queen Yun, served Yeonsangun's father, King Seongjong, as a concubine until the death of Queen Gonghye, Seongjong's first wife. With no heir, the king was urged by counselors to take a second wife to secure the royal succession. Lady Yun was chosen for her beauty and was formally married in 1476. Several months later, she gave birth to Yi Yung.

The new queen proved to be temperamental and highly jealous of the other concubines, even going as far as poisoning one of them in 1477. One night in 1479, she physically struck her husband (the king) and left scratch marks. Despite his efforts to conceal the injury, Seongjong's mother, Royal Queen Dowager Insu, discovered the truth and ordered Lady Yun into exile. After several attempts to restore the deposed queen to her former position, government officials petitioned that she be executed. Queen Yun died in 1482, after being ordered to commit suicide by drinking poison.

The crown prince grew up believing that he was the son of Queen Jeonghyeon, his father's third wife. He succeeded to the throne in 1495 and during his early reign, he was a wise and able administrator who strengthened the national defense and aided the poor. However, he also showed signs of a violent side when he killed Jo Sa-seo, one of his tutors, soon after becoming king.

Yeonsangun eventually learned the truth about his biological mother and attempted to posthumously restore her titles and position. However, government officials belonging to the Sarim faction opposed his efforts on account of serving Seongjong's will, and greatly displeased, Yeonsangun started looking for ways to eliminate them.

In 1498, Kim Il-son [ko], a disciple of Kim Chong-jik, included a paragraph in the royal records that was critical of King Sejo's coup d'état (1455). Kim Il-son and other followers of Kim Chong-jik were accused of treason by a rival faction, giving the king cause enough to order the execution of many Sarim officials and the mutilation of Kim Chong-jik's remains. This came to be known as the First Literati Purge (Muo Sahwa; 무오사화, 戊午士禍).

In 1504, Im Sa-hong revealed to Yeonsangun the details of his mother's death and showed him a blood-stained piece of clothing that was obtained from Lady Yun's mother, the blood allegedly vomited by her after taking poison.

Subsequently, Yeonsangun beat to death two of his father's concubines, Lady Jeong and Lady Eom, for their part in his mother's death. His grandmother, Grand Royal Queen Dowager Insu, also died soon after he pushed her during an altercation. He sentenced to death many government officials who had supported the execution of his mother, now posthumously honored as "Queen Jeheon" (제헌왕후; 齊獻王后), and ordered the grave of Han Myŏnghoe to be opened and the head to be cut off the corpse.