Arguably the oldest active political party in Brazil, it played an important role in the country's 20th-century history despite the relatively small number of members.

The Brazilian Communist Party (in Portuguese, Partido Comunista Brasileiro), abbreviated as the PCB, was founded on 25 March 1922 in the city of Niterói, Rio de Janeiro.

On that day, nine representatives of communist groups from the cities of São Paulo, Santos, Cruzeiro, Porto Alegre, Recife, Niterói, Juiz de Fora and Rio de Janeiro met and approved the party's statutes and the twenty-one conditions for entering the Communist International, though the PCB was not recognized by the Comintern in its first years due to its eclectic ideological roots.

Inspired by the Russian Revolution, a group of former anarchist militants who were disappointed in the lack of unity and force of the movement, turned communist and started the PCB.

At the same time, some other figures from Brazil's early labour and socialist movements became inspired by forms of military and authoritarian populism, like the tenentes, that eventually led to the Vargas-revolution of 1930.

This marks the beginning of a long period of submission to, initially the Third International, and, after its dissolution, to the political leadership of the Communist Party of the Soviet Union.

In 1943, during the so-called Mantiqueira Conference, the party secretly met in the small city of Engenheiro Passos, Rio de Janeiro, and in an open letter to Vargas decided to support a declaration of war on the Axis.

However, this period of official tolerance did not last long, as President Dutra denounced the PCB as "internationalist, and therefore not committed to Brazil's own interests" in 1948, an action supported by the American government.

PCB did play an important role against the dictatorship, by organizing the workers movement and participating in efforts to unite the opposition in its demands for democratic reforms.

Forswearing the Popular Front strategy adopted by the PCB, the PT chose instead to take a more radical stance against the military regime and gained a strong presence in between high skilled workers and middle-class intellectuals.

The 1982 Congress confirmed its democratic agenda, declaring the PCB "a party of the masses, linking socialist goals to true democracy, which will be constructed based on the values of freedom".

It remains small, in spite of some recent growth during the government of Luiz Inácio Lula da Silva and the crisis of PT (it should be noticed that the main result of this process was the Socialism and Liberty Party).

Putting aside some significant ideological differences, the three parties built a common agenda to try to break the polarity between presidential candidates Luiz Inácio Lula da Silva (PT) and Geraldo Alckmin (PSDB).

The PCB elected 46 state deputies and become a major party in the Federal District (at this time, the city of Rio de Janeiro).