The 1955 State of Vietnam referendum determined the future form of government of the State of Vietnam, the nation that was to become the Republic of Vietnam (widely known as South Vietnam). It was contested by Prime Minister Ng nh Dim, who proposed a republic, and former emperor Bo i, who had abdicated in 1945 and at the time of the referendum held the title of head of state.

Though published counts showed Dim winning the election with 98.9% of the vote, the referendum was widely marred by electoral fraud. In the capital, Saigon, Dim was credited with more than 600,000 votes, although only 450,000 people were on the electoral roll, while Dim was also credited with receiving over 90% of registered voters in rural regions where opposition groups had prevented voting.

The referendum was the last phase in the power struggle between Bo i and his prime minister. Bo i disliked Dim and had frequently attempted to undermine him, having appointed him only because he was a conduit to American aid. At the time, the country was going through a period of insecurity, as Vietnam had been temporarily partitioned as a result of the 1954 Geneva Accords that ended the First Indochina War. The State of Vietnam controlled the southern half of the country, pending national elections that were intended to reunify the country under a common government. Still, the Vietnamese National Army was not in full control of southern Vietnam; the Cao i and Ha Ho religious sects ran their own administrations in the countryside supported by private armies, while the Bnh Xuyn organised crime syndicate controlled the streets of Saigon. Despite interference from these groups, Bo i, and even French officials, Dim managed to subdue the private armies and consolidate government control over the country by mid-1955.

Emboldened by his success, Dim began to plot Bo i's downfall. He scheduled a referendum for 23 October 1955 and pushed Bo i out of the political scene, hindering the former emperor's attempts to derail the poll. In the period leading up to the vote, campaigning for Bo i was banned, while Dim's election campaign focused on personal attacks against Bo i. These included pornographic cartoons of the head of state and unverified rumours claiming he was illegitimate and linking him to various mistresses. The government-controlled media launched polemical attacks on Bo i, and police went door-to-door, warning people of the consequences of failing to vote. After his brother Ng nh Nhu had rigged the referendum, Dim proclaimed himself president of the newly created Republic of Vietnam.

Ngô Đình Diệm ( or ; Vietnamese: [ŋō ɗìn jîəmˀ] (listen); 3 January 1901 – 2 November 1963) was a Vietnamese politician. He was the final prime minister of the State of Vietnam (1954–1955), and then served as President of South Vietnam (Republic of Vietnam) from 1955 until he was captured and assassinated during the 1963 military coup.

Diệm was born into a prominent Catholic family, the son of a high-ranking civil servant, Ngô Đình Khả. He was educated at French-speaking schools and considered following his brother Ngô Đình Thục into the priesthood, but eventually chose to pursue a civil-service career. He progressed rapidly in the court of Emperor Bảo Đại, becoming governor of Bình Thuận Province in 1929 and interior minister in 1933. However, he resigned the latter position after three months and publicly denounced the emperor as a tool of France. Diệm came to support Vietnamese nationalism, promoting an anti-communist and anti-colonialist "third way" opposed to both Bảo Đại and communist leader Hồ Chí Minh. He established the Can Lao Party to support his political doctrine of Person Dignity Theory.

After several years in exile, Diệm returned home in July 1954 and was appointed prime minister by Bảo Đại, the head of the Western-backed State of Vietnam. The Geneva Accords were signed soon after he took office, formally partitioning Vietnam along the 17th parallel. Diệm soon consolidated power in South Vietnam, aided by his brother Ngô Đình Nhu. After a rigged referendum in 1955, he proclaimed the creation of the Republic of Vietnam, with himself as president. His government was supported by other anti-communist countries, most notably the United States. Diệm pursued a series of nation-building schemes, emphasising industrial and rural development. From 1957, he was faced with a communist insurgency backed by North Vietnam, eventually formally organized under the banner of the Việt Cộng. He was subject to a number of assassination and coup attempts, and in 1962 established the Strategic Hamlet Program as the cornerstone of his counterinsurgency effort.

Diệm's favoritism towards Catholics and persecution of South Vietnam's Buddhist majority led to the "Buddhist crisis" of 1963. The violence damaged relations with the United States and other previously sympathetic countries, and his regime lost favour with the leadership of the Army of the Republic of Vietnam. On 1 November 1963, the country's leading generals launched a coup d'état with assistance from the CIA. He and his younger brother Nhu initially escaped, but were recaptured the following day and assassinated on the orders of Dương Văn Minh, who succeeded him as president. Diệm has been a controversial historical figure in historiography on the Vietnam War. Some historians have considered him a tool of the United States, while others portrayed him as an avatar of Vietnamese tradition. At the time of his assassination, he was widely considered to be a corrupt dictator.