South Vietnam was a state which governed southern Vietnam until 1975. It received international recognition in 1950 as the State of Vietnam (1949–55) and later as the Republic of Vietnam (1955–75). Its capital was Saigon. The terms South Vietnam and North Vietnam became common usage in 1954 at the time of the Geneva Conference, which partitioned Vietnam into communist and non-communist zones at the 17th parallel. The United States was an ally of South Vietnam during the Vietnam War.

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.

Vietnam ( listen), formally the Socialist Republic of Vietnam (Cộng hòa xã hội chủ nghĩa Việt Nam;  listen), is the easternmost country on the Indochina Peninsula in Southeast Asia. With an estimated 91.5 million inhabitants as of 2012, it is the world's 13th-most-populous country, and the eighth-most-populous Asian country. The name Vietnam translates as "South Viet", and was officially adopted in 1945. The country is bordered by China to the north, Laos to the northwest, Cambodia to the southwest, and the South China Sea to the east.

The Vietnamese became independent from Imperial China in 938 AD, following the Battle of Bạch Đằng River. Successive Vietnamese royal dynasties flourished as the nation expanded geographically and politically into Southeast Asia, until the Indochina Peninsula was colonized by the French in the mid-19th century. The First Indochina War eventually led to the expulsion of the French in 1954, leaving Vietnam divided politically into two states, North and South Vietnam. Conflict between the two sides intensified, with heavy foreign intervention, during the Vietnam War, which ended with a North Vietnamese victory in 1975.