The party was founded in 1921 as the Portuguese section of the Communist International (Comintern). Made illegal after a coup in the late 1920s, the PCP played a major role in the opposition to the dictatorial regime of António de Oliveira Salazar.
Despite being less influential since the fall of the Socialist bloc in eastern Europe, the party still enjoys popularity in large sectors of Portuguese society, particularly in the rural areas of the Alentejo and Ribatejo, and in the heavily industrialized areas around Lisbon and Setúbal.