Pietro Sandro Nenni (Italian: [ˈpjɛːtro ˈnɛnni]; 9 February 1891 – 1 January 1980) was an Italian socialist politician and statesman, the national secretary of the Italian Socialist Party (PSI) and senator for life since 1970. He was a recipient of the Stalin Peace Prize in 1951. He was one of the founders of the Italian Republic and a central figure of the Italian political left from the 1920s to the 1960s.

Nenni was born in Faenza, in Emilia-Romagna. After his peasant parents died, he was placed in an orphanage by an aristocratic family. Every Sunday, he recited his catechism before the countess and if he did well, he received a silver coin, which he recalled as "generous but humiliating".

Nenni was affiliated with the Italian Republican Party. In 1908, he became editor of a republican paper in Forlì. The socialist paper in the town was edited at the time by Benito Mussolini, later the Fascist dictator of Italy. In 1909 he entered political life by joining the Italian Republican party. Nenni was arrested in 1911 for his participation in a socialist protest against Italy's imperialistic war in Libya alongside Mussolini and was imprisoned for seven months.

When the First World War broke out, Nenni advocated the intervention of Italy in the war. In 1915, he volunteered for the Isonzo front. After he was wounded and sent home, he became an editor of the republican paper Mattine d'Italia. He defended Italy's participation in the war but tried not to alienate his socialist friends. In the last years of the war Nenni served at the front again.

When the war was over, Nenni founded, together with some disillusioned revolutionary ex-servicemen, a group called "Fascio", which was soon dissolved and replaced by a real Fascist body. While the socialist Mussolini became a fascist, the republican Nenni joined the Socialist Party in 1921 after its split with the wing that would form the Italian Communist Party (PCI).

In 1923, after the Fascist March on Rome, Nenni became the editor of PSI's official organ, Avanti!, and engaged in antifascist activism. In 1925 he was arrested for publishing a booklet on the fascist murder of Socialist leader Giacomo Matteotti. When the Avanti offices were set aflame and the paper prohibited in 1926, he took refuge in France, where he became secretary of the PSI.

Nenni had worked in Paris as a correspondent of the Avanti in 1921 and had become acquainted with Léon Blum, Marcel Cachin, Romain Rolland and Georges Sorel. During his Parisian exile, Nenni made a decisive contribution to the survival of the Italian Socialist Party, which had moved abroad, and he worked for an alliance between the various anti-fascist parties which had been driven into exile. In 1935, he helped lead the Italian opposition to Mussolini's invasion of Ethiopia. Nenni went on to fight with the International Brigades in the Spanish Civil War. He was the co-founder and the political commissar of the Garibaldi Brigade. After the defeat of the Spanish Republic and the victory of General Francisco Franco he returned to France. In 1943, he was arrested by the Germans in Vichy France and then imprisoned in Italy on the island of Ponza.

Nenni's third daughter, Vittoria, was active in the French resistance. She was captured and deported to Auschwitz, where she was murdered on 15 July 1943, aged 28. After being liberated in August 1943, he returned to Rome to lead the Italian Socialist Party, which had been reunified as the Italian Socialist Party of Proletarian Unity. After the surrender of Italy with the Allied armed forces on 8 September 1943, he was one of the political officials of the National Liberation Committee, the underground political entity of Italian Partisans during the German occupation.