The Italian Socialist Party (Italian: Partito Socialista Italiano, PSI) is a social-democratic political party in Italy. The party was founded in 2007–2008 by the merger of the following social-democratic parties and groups: Enrico Boselli's Italian Democratic Socialists (legal successor of the Italian Socialist Party), the faction of the New Italian Socialist Party led by Gianni De Michelis, The Italian Socialists of Bobo Craxi, Democracy and Socialism of Gavino Angius, the Association for the Rose in the Fist of Lanfranco Turci, Socialism is Freedom of Rino Formica and some other minor organisations. Until October 2009, the party was known as Socialist Party (Italian: Partito Socialista, PS).

From 2008 to 2019, Riccardo Nencini from Tuscany has been party leader. Elected senator with the Democratic Party in 2013 and re-elected in 2018, he was Deputy Minister of Infrastructures and Transports from 2014 to 2019 (Renzi Cabinet and Gentiloni Cabinet). In March 2019, Nencini stepped down as secretary and was replaced by Enzo Maraio, from Campania. Between 2019 and 2022, the PSI sat within the parliamentary group of Italia Viva in the Senate. In the 2022 general election the party lost its parliamentary representation.

A merger of all the parties hailing from the Italian Socialist Party (PSI), disbanded in 1994 as a result of Tangentopoli scandals, and the Italian Democratic Socialist Party (PSDI) was initially proposed by Enrico Boselli during a congress of his Italian Democratic Socialists (SDI) in April 2007. In the event, the party decided not to join the Democratic Party (PD), aiming for a "Socialist Constituent Assembly" (Costituente Socialista) instead.

Some minor parties and associations, including The Italian Socialists of Bobo Craxi, Socialism is Freedom of Rino Formica and the Association for the Rose in the Fist of Lanfranco Turci, immediately welcomed Boselli's proposal, while in June 2007 the New Italian Socialist Party (NPSI) divided in two groups: the first, led by Stefano Caldoro and Lucio Barani, opted to join The People of Freedom and retained the NPSI identity; the second, led by Mauro Del Bue and Gianni De Michelis, agreed to join the Socialist Constituent Assembly. Indeed, the two factions elected their secretary in two separate congresses: on 24 June Caldoro was elected secretary by the right-wing faction of the party, while on 7–8 July Del Bue and De Michelis were elected respectively secretary and president by those members who wanted to take part to the foundation of the Socialist Party, whose first meeting took place on 14 July. In practice, for some time the NPSI had two secretaries. Subsequently, on 24 October 2007, the two MEPs elected on the United Socialists for Europe list in the 2004 European Parliament election, De Michelis and Alessandro Battilocchio, joined the Socialist Group in the European Parliament. Battilocchio would later split, re-regroup in the new NPSI and finally join Forza Italia.

At its foundation in October 2007, the Socialist Party (PS) was joined also by Gavino Angius's Democracy and Socialism, a group of former Democrats of the Left most recently affiliated with the Democratic Left.

At the 2008 general election, the PS stood alone and Boselli was its candidate for Prime Minister. In the event, the party gained less than 1% of the vote (well below thresholds) and failed to win any seats in the Italian Parliament.

At the first party congress, which took place in July 2008, Riccardo Nencini was elected secretary (replacing Boselli) while Pia Locatelli was elected president. In September, Nencini proposed a new "reformist axis" comprising also the Democratic Party (PD) and the Union of the Centre (UdC), while explaining that the Democrats needed to choose between "reformism" and the "populism" of Italy of Values (IdV). In October 2008, Angius led his group into the PD, proposing that the entire PS should follow him. In reply, Nencini underlined that no former members of the late PSI were leaving the party. In fact, Spini, the only former Socialist in Angius' group, chose to stay in the PS. However, De Michelis left the party soon after.

In the 2009 European Parliament election, the PS formed a joint list named Left and Freedom (SL) with the Movement for the Left (MpS), the Federation of the Greens (FdV), the Democratic Left (SD) and Unite the Left (UlS). SL obtained 3.1% of the vote and no MEPs. Despite this, the PS' national council chose to continue to organise a "secular, libertarian and left-wing" force. This led to the exit of Craxi, who launched the United Socialists in October. However, one month later the PS left SL, later rebranded Left Ecology Freedom (SEL), having refused a full merger. The PS was renamed Italian Socialist Party (PSI), chose to field party lists in support of PD candidates in the forthcoming regional elections, opening the way to Craxi's return.