The Unitary Socialist Party (Partito Socialista Unitario, PSU) was a democratic socialist political party in Italy active from 1922 to 1930.

The party was founded in November 1922 by the reformist wing of the Italian Socialist Party (PSI) led by Rinaldo Rigola, Filippo Turati, Vittorio Emanuele Modigliani, Giacomo Treves, and Giacomo Matteotti, after they had been expelled in October.

[1][2] A staunch opponent of Benito Mussolini and Italian fascism,[3] Matteotti was assassinated by Fascists, affiliated to OVRA, in June 1924.

Outlawed in November 1925, the PSU remained active as the clandestine Italian Workers' Socialist Party (Partito Socialista dei Lavoratori Italiani; PSLI).

Leading members and activists of the party included Oddino Morgari, Sandro Pertini, Camillo Prampolini, Claudio Treves and Anna Kulischov.