What was the official name of South Vietnam?

The official name of South Vietnam was the Republic of Vietnam.

The Republic of Vietnam emerged after the 1954 partition of Vietnam, when the Geneva settlement placed the communist Democratic Republic of Vietnam in the north and the State of Vietnam in the south. In October 1955, Ngô Đình Diệm removed Bảo Đại as head of state and proclaimed a republic, adopting this name.

“South Vietnam” is the common English geographical and political label, while “Republic of Vietnam” was the state’s formal name. It is therefore not the same as the Democratic Republic of Vietnam, the official name used by North Vietnam. The earlier State of Vietnam was its predecessor, not its final official title.

The Republic of Vietnam ended when communist forces captured Saigon on April 30, 1975. In 1976, North Vietnam and the Republic of South Vietnam merged to create the Socialist Republic of Vietnam.