The PSIUP was formed on 12 January 1964 by a leftist section of the Italian Socialist Party (PSI).

Other leading members were Lelio Basso, Vittorio Foa, Lucio Libertini, Emilio Lussu, Francesco Cacciatore detto Cecchino and Dario Valori.

[2] Instead, the founders of the PSIUP favoured cooperation with the Italian Communist Party (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).