The State of Vietnam referendum of 1955 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 Ngô Đình Diệm, who proposed a republic, and former emperor Bảo Đại. Bảo Đại had abdicated as emperor in 1945 and at the time of the referendum held the title of head of state. Though published counts showed Diệm winning the election with 98.2% of the vote, the referendum was widely marred by electoral fraud. In the capital, Saigon, Diệm was credited with more than 600,000 votes, although only 450,000 people were on the electoral roll. He accumulated tallies in excess of 90% of the registered voters, even in rural regions where opposition groups prevented voting.
The referendum was the last phase in the power struggle between Bảo Đại and his prime minister. Bảo Đại disliked Diệm 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 Đài and Hòa Hảo religious sects ran their own administrations in the countryside supported by private armies, while the Bình Xuyên organised crime syndicate controlled the streets of Saigon. Despite interference from these groups, Bảo Đại, and even French officials, Diệm managed to subdue the private armies and consolidate government control over the country by mid-1955.
Emboldened by his success, Diệm began to plot Bảo Đại's downfall. He scheduled a referendum for 23 October 1955 and pushed Bảo Đại 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 Bảo Đại was banned, while Diệm's election campaign focused on personal attacks against Bảo Đại. 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 Bảo Đại, and police went door-to-door, warning people of the consequences of failing to vote. After his brother Ngô Đình Nhu rigged the poll, Diệm proclaimed himself president of the newly created Republic of Vietnam.