The Italian Social Republic (Italian: Repubblica Sociale Italiana, IPA: [reˈpubblika soˈtʃaːle itaˈljaːna]; RSI; German: Italienische Sozialrepublik, IPA: [itaˈli̯eːnɪʃə zoˈtsi̯aːlʁepuˌbliːk]), known prior to December 1943 as the National Republican State of Italy (Italian: Stato Nazionale Repubblicano d'Italia; SNRI), but more popularly known as the Republic of Salò (Italian: Repubblica di Salò, IPA: [reˈpubblika di saˈlɔ]), was a German puppet state and fascist rump state with limited diplomatic recognition that was created during the latter part of World War II. It was a collaborationist regime in German-occupied Italy, established after the German invasion of Italy in September 1943 and disbanded with the surrender of Axis troops in Italy in May 1945. The German occupation triggered widespread national resistance against it and the Italian Social Republic during the liberation of Italy and the Italian civil war.

The Italian Social Republic was the second and last incarnation of the Italian Fascist state, led by the Italian dictator Benito Mussolini and his reformed anti-monarchist Republican Fascist Party. The newly founded state declared Rome its capital but the de facto capital was Salò (hence the colloquial name of the state), a small town on Lake Garda, near Brescia, where Mussolini and the Ministry of Foreign Affairs were headquartered. The Italian Social Republic nominally exercised sovereignty in Northern and Central Italy, but was largely dependent on German troops to maintain control.

In July 1943, after the Allies had pushed Italy out of North Africa and subsequently invaded Sicily, the Grand Council of Fascism—with the support of King Victor Emmanuel III—overthrew and arrested Mussolini. The new government began secret peace negotiations with the Allied powers but when the Armistice of Cassibile was announced on 8 September, Nazi Germany was prepared and quickly intervened. German troops seized control of the northern half of Italy, freed Mussolini, and brought him to the German-occupied area to establish a satellite regime. The Italian Social Republic was proclaimed on 23 September 1943. Although the RSI claimed sovereignty over all of Italy and its colonies, its de facto jurisdiction only extended to a vastly reduced portion of the country. The RSI received diplomatic recognition only from the Axis powers and their satellite states. Finland and Vichy France, although in the German orbit, did not recognize it. Unofficial relations were maintained with Argentina, Portugal, Spain, and, through commercial agents, Switzerland. Vatican City did not recognize the RSI.

Around 25 April 1945, 19 months after its founding, the RSI all but collapsed. In Italy, the day is known as Liberation Day (festa della liberazione). On that day, a general partisan uprising, alongside the efforts of Allied forces during their final offensive in Italy, managed to oust the Germans and the remaining RSI forces from Italy almost entirely. Mussolini was captured and killed by Italian partisans on 28 April as he and an entourage attempted to flee. The RSI Minister of Defense, Rodolfo Graziani, surrendered what was left of the Italian Social Republic on 1 May, one day after the German forces in Italy capitulated.

On 24 July 1943, after the Allied landings in Sicily, on a motion by Dino Grandi, the Grand Council of Fascism voted a motion of no confidence in Prime Minister Benito Mussolini. Mussolini's authority had been undermined by a series of military defeats from the start of Italy's entry into the war during June 1940, including the bombing of Rome, the loss of the African colonies in the East African and North African campaigns, the defeat of the Italian Army in Russia and the Allied invasion of Sicily.

The next day, King Victor Emmanuel III dismissed Mussolini from office, ordered him arrested, and appointed Marshal Pietro Badoglio as new prime minister. By this time, the monarchy, a number of Fascist government members, and the general Italian population had grown tired of the futile war effort which had driven Italy into subordination to and subjugation by Nazi Germany. The failed war effort left Mussolini humiliated at home and abroad as a "sawdust Caesar". The new government began secret negotiations with the Allied powers and made preparations for the capitulation of Italy. These surrender talks implied a commitment from Badoglio to leave the Axis alliance.

While the Germans formally recognised the new status quo in Italian politics, they intervened by sending some of the best units of the Wehrmacht to Italy. This was done both to resist new Allied advances and to face the predictably imminent defection of Italy. While Badoglio continued to swear loyalty to Germany and the Axis powers, Italian government emissaries prepared to sign an armistice at Cassibile in Allied-occupied Sicily, which was finalized on 3 September.

On 8 September, Badoglio announced Italy's armistice with the Allies (although termed an "armistice", its terms made it akin to an unconditional surrender). German Führer Adolf Hitler and his staff, long aware of the negotiations, acted immediately by ordering German troops to seize control of Northern and Central Italy. The Germans quickly occupied Italy, disarmed the Italian troops and took over all of the Italian Army's materials and equipment, meeting only limited resistance. The Germans also dissolved the Italian occupation zone in southeastern France and forced Italian troops stationed there to leave. The Italian armed forces were not given clear orders to resist the Germans after the armistice and so resistance to the German takeover was scattered and of little effect. King Victor Emmanuel made no effort to rally resistance to the Germans, instead fleeing with his retinue to the safety of the Allied lines. On 10 September 1943, after two days of battle between the Wehrmacht and the remnants of the Royal Italian Army, Rome fell to the Germans.