An institutional referendum (Italian: referendum istituzionale, or referendum sulla forma istituzionale dello Stato) was held by universal suffrage in the Kingdom of Italy on 2 June 1946, a key event of contemporary Italian history. Until 1946, Italy was a kingdom ruled by the House of Savoy, reigning since the unification of Italy in 1861 and previously rulers of the Kingdom of Sardinia. In 1922, the rise of Benito Mussolini and the creation of the Fascist regime in Italy, which eventually resulted in engaging the country in World War II alongside Nazi Germany, considerably weakened the role of the royal house.

Following the Italian Civil War and the Liberation of Italy from Axis troops in 1945, a popular referendum on the institutional form of the state was called the next year and resulted in voters choosing the replacement of the monarchy with a republic. The 1946 Italian general election to elect the Constituent Assembly of Italy was held on the same day. As with the simultaneous Constituent Assembly elections, the referendum was not held in the Julian March, in the province of Zara or the province of Bolzano, which were still under occupation by Allied forces pending a final settlement of the status of the territories.

The results were proclaimed by the Supreme Court of Cassation on 10 June 1946: 12,717,923 citizens in favor of the republic and 10,719,284 citizens in favor of the monarchy. The event is commemorated annually by the Festa della Repubblica. The former King Umberto II voluntarily left the country on 13 June 1946, headed for Cascais, in southern Portugal, without even waiting for the results to be defined and the ruling on the appeals presented by the monarchist party, which were rejected by the Supreme Court of Cassation on 18 June 1946. With the entry into force of the new Constitution of the Italian Republic, on 1 January 1948, Enrico De Nicola became the first to assume the functions of president of Italy. It marked the first time that most of the Italian Peninsula was under a single republican government since the fall of the Roman Republic.

In the history of Italy there are several so-called "republican" governments that have followed one another over time. Examples are the ancient Roman Republic and the medieval maritime republics. From Cicero to Niccolò Machiavelli, Italian philosophers have imagined the foundations of political science and republicanism. But it was Giuseppe Mazzini who revived the republican idea in Italy in the 19th century.

An Italian nationalist in the historical radical tradition and a proponent of a republicanism of social-democratic inspiration, Mazzini helped define the modern European movement for popular democracy in a republican state. Mazzini's thoughts had a very considerable influence on the Italian and European republican movements, in the Constitution of Italy, about Europeanism and more nuanced on many politicians of a later period, among them American president Woodrow Wilson, British prime minister David Lloyd George, Mahatma Gandhi, Israeli prime minister Golda Meir and Indian prime minister Jawaharlal Nehru. Mazzini formulated a concept known as "thought and action" in which thought and action must be joined and every thought must be followed by action, therefore rejecting intellectualism and the notion of divorcing theory from practice.

In July 1831, in exile in Marseille, Giuseppe Mazzini founded the Young Italy movement, which aimed to transform Italy into a unitary democratic republic, according to the principles of freedom, independence and unity, but also to oust the monarchic regimes pre-existing the unification, including the Kingdom of Sardinia. The foundation of the Young Italy constitutes a key moment of the Italian Risorgimento and this republican program precedes in time the proposals for the unification of Italy of Vincenzo Gioberti and Cesare Balbo, aimed at reunifying the Italian territory under the presidency of the Pope. Subsequently, the philosopher Carlo Cattaneo promoted a secular and republican Italy in the extension of Mazzini's ideas, but organized as a federal republic.

The political projects of Mazzini and Cattaneo were thwarted by the action of the Piedmontese Prime Minister Camillo Benso, Count of Cavour, and Giuseppe Garibaldi. The latter set aside his republican ideas to favor Italian unity. After having obtained the conquest of the whole of southern Italy during the Expedition of the Thousand, Garibaldi handed over the conquered territories to the king of Sardinia Victor Emmanuel II, which were annexed to the Kingdom of Sardinia after a plebiscite. This earned him heavy criticism from numerous republicans who accused him of treason. While a laborious administrative unification began, a first Italian parliament was elected and, on 17 March 1861, Victor Emmanuel II was proclaimed king of Italy.

From 1861 to 1946, Italy was a constitutional monarchy founded on the Albertine Statute, named after the king who promulgated it in 1848, Charles Albert of Sardinia. The parliament included a Senate, whose members were appointed by the king, and a Chamber of Deputies, elected by census vote. In 1861 only 2% of Italians had the right to vote. In the political panorama of the time there was a republican political movement which had its martyrs, such as the soldier Pietro Barsanti. Barsanti was a supporter of republican ideas, and was a soldier in the Royal Italian Army with the rank of corporal. He was sentenced to death and shot in 1870 for having favored an insurrectional attempt against the Savoy monarchy and is therefore considered the first martyr of the modern Italian Republic and a symbol of republican ideals in Italy.