South Vietnam, officially the Republic of Vietnam, was a state which governed southern Vietnam until 1975. It received international recognition in 1949 as the "State of Vietnam" (1949–55), and later as the "Republic of Vietnam" (1955–75). Its capital was Saigon. The term "South Vietnam" became common usage in 1954, when the Geneva Conference partitioned Vietnam into communist and non-communist parts.

South Vietnam's origins can be traced to the French colony of Cochinchina, which consisted of the southern third of Vietnam and was a subdivision of French Indochina. After World War II, the Vietminh, led by Ho Chi Minh, proclaimed the establishment of a Communist nation in Hanoi. In 1949, non-communist Vietnamese politicians formed a rival government in Saigon led by former emperor Bao Dai. Bao Dai was deposed by Prime Minister Ngo Dinh Diem in 1955, who proclaimed himself president after a referendum. After Diem was deposed in a military coup in 1963, there was a series of short-lived military governments. General Nguyen Van Thieu led the country from 1967 until 1975. The Vietnam War began in 1959 with an uprising by Viet Cong forces supplied by North Vietnam. Fighting climaxed during the Tet Offensive of 1968, when there were over 1.5 million South Vietnamese soldiers and 500,000 U.S. soldiers in South Vietnam. Despite a peace treaty concluded in January 1973, fighting continued until the North Vietnamese army overran Saigon on 30 April 1975.