The Jeju uprising, known in South Korea as the Jeju April 3 incident (Korean: 43 ), was an uprising on Jeju Island from April 1948 to May 1949. Residents of Jeju opposed to the division of Korea had protested and had been on a general strike since 1947 against elections scheduled by the United Nations Temporary Commission on Korea (UNTCOK) to be held only in the territory controlled by the United States Army Military Government in Korea. The Workers' Party of South Korea (WPSK) and its supporters launched an insurgency in April 1948, attacking the police, and Northwest Youth League members stationed on Jeju mobilized to violently suppress the protests.:166167 The First Republic of Korea under President Syngman Rhee escalated the suppression of the uprising from August 1948, declaring martial law in November and beginning an "eradication campaign" against rebel forces in the rural areas of Jeju in March 1949, defeating them within two months. Many rebel veterans and suspected sympathizers were later killed upon the outbreak of the Korean War in June 1950, and the existence of the Jeju uprising was officially censored and repressed in South Korea for several decades.:41The Jeju uprising was notable for its extreme violence; between 14,000 and 30,000 people (10 percent of Jeju's population) were killed, and 40,000 fled to Japan.:139,193 Atrocities and war crimes were committed by both sides, but historians have noted that the methods used by the South Korean government to suppress protesters and rebels were especially cruel, with violence against civilians by pro-government forces contributing to the Yeosu-Suncheon rebellion in South Jeolla during the conflict.:171:1314:186 Some historians and scholars, including military historian Allan R. Millett, regard the Jeju uprising as the true beginning of the Korean War.In 2006, almost 60 years after the Jeju uprising, the South Korean government apologized for its role in the killings and promised reparations. In 2019, the South Korean police and defense ministry apologized for the first time over the massacres.

Jeju Province, officially Jeju Special Self-Governing Province, is one of the nine provinces of South Korea. The province comprises Jeju Island (Korean: 제주도; RR: Jejudo; IPA: [t͡sed͡zudo]), formerly transliterated as Cheju or Cheju Do, the country's largest island. It was previously known as Quelpart to Europeans and during the Japanese occupation as Saishū. The island lies in the Korea Strait, southwest of South Jeolla Province, of which it was a part before it became a separate province in 1946. Its capital is Jeju City and it is home to South Korea's tallest mountain, Mt. Halla.