The origins of the PSTU can be traced to 1972 when a number of Brazilian militants from a variety of political backgrounds contacted the reunified Fourth International (FI).

In 1978, the Workers League formed the Movimento Convergência Socialista (CS) as part of a tactic aimed at the establishment of a new mass socialist party.

By 1979, CS was part of a growing campaign for a new Workers' Party and played an important role in the metal working strikes that took place in the ABC region that year.

In the early 1980s, CS took part in the founding of the Central Unica dos Trabalhadores (CUT) union centre as strikes spread through Brazil along with other forms of social unrest.

The PSTU publishes the weekly newspaper Opinião Socialista (Socialist Opinion) and is the main group contributing to the journal Marxism Alive/Marxismo Vivo.

The PSTU is also the main force of the Brazilian extreme left and first took part in elections in the Third Pole that supported the candidacy of Heloísa Helena Lima de Moraes Carvalho, former member of the PT (the party of Luiz Inácio Lula da Silva) expelled from the PT with others for her opposition to the reform of pensions (in 2008 she became leader of the Socialism and Liberty Party with Ivan Valente and Luciana Genro).