The Unified PSI–PSDI (PSI–PSDI Unificati) was the name of the federation of parties formed by the Italian Socialist Party (PSI) and the Italian Democratic Socialist Party (PSDI) from 1966 to 1969. The federal party was often unofficially referred to as the Unified Socialist Party (Partito Socialista Unificato).
The two parties joined forces in 1966, after the entrance of PSI for the first time in an Italian government majority, the centre-left coalition led by Aldo Moro and composed of Christian Democracy, Italian Republican Party and PSDI. The membership was 700,964 activists.
After the unsuccessful performance of the united party in the 1968 general election, the PSi-PSDI returned officially to the name PSI. On July 6, 1969, the former members of PSDI left to form the "United Socialist Party", which adopted back the denomination of PSDI in 1971.