The Italian Socialist Party of Proletarian Unity (Partito Socialista Italiano di Unità Proletaria, PSIUP) was a political party in Italy, active from 1964 to 1972.

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Italian Socialist Party of Proletarian Unity Partito Socialista Italiano di Unità Proletaria
Abbreviation	PSIUP
President	Lelio Basso
General Secretary	Tullio Vecchietti
Spokesperson	Dario Valori
Founded	12 January 1964 (1964-01-12)
Dissolved	13 July 1972 (1972-07-13)
Split from	Italian Socialist Party
Merged into	Italian Communist Party
Headquarters	Rome, Italy
Newspaper	Mondo nuovo
Ideology	Socialism Marxism Frontism Factions Pro-Soviet Union Revolutionary socialism Libertarian socialism
Political position	Left-wing Factions Far-left
Colours	Red
Politics of Italy Political parties Elections

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The PSIUP was formed on 12 January 1964 by a leftist section of the Italian Socialist Party (PSI).^[1] PSIUP had been the PSI's name in 1943–1947. The new PSIUP was led by Tullio Vecchietti. Other leading members were Lelio Basso, Vittorio Foa, Lucio Libertini, Emilio Lussu, Francesco Cacciatore detto Cecchino and Dario Valori. The new party attracted PSI militants who were dissatisfied with the close cooperation between the PSI and the Christian Democracy.^[2] Instead, the founders of the PSIUP favoured cooperation with the Italian Communist Party (PCI).

On 13 July 1972, following a disappointing electoral result, the PSIUP split. The majority, led by Libertini, Valori and Vecchietti, joined the PCI. The rightist minority, led by Giuseppe Avolio, Nicola Corretto and Vincenzo Gatto, rejoined the PSI. The leftist minority, led by Foa and Silvano Miniati, continued to work under the name PSIUP, and in December 1972 they established the Proletarian Unity Party (PdUP). A Posadist faction within the PSIUP published the Bollettino della sinistra rivoluzionaria del PSIUP between 1965 and 1967.

• Tullio Vecchietti (January 1964 – September 1971)
• Dario Valori (October 1971 – July 1972)

• Giuseppe Impastato, anti-mafia journalist member of the party, assassinated on 9 May 1978