It was formed by the fusion of the Trotskyist Communist Left of Spain (Spanish: Izquierda Comunista de España, ICE) and the Workers and Peasants' Bloc (BOC, affiliated with the Right Opposition) against the will of Leon Trotsky, with whom the former broke.

In his writings on the Spanish Revolution, Trotsky would elaborate on his overall criticisms of the POUM such as their abandonment of the Left Opposition program in favour of reformism to retain tactical advantage among other political tendencies.

[2] The POUM's independent communist position, including opposition to Stalin, caused huge ruptures with the PCE, which remained fiercely loyal to the Comintern.

The POUM was a member of the London Bureau of socialist and centrist Marxist parties that rejected both the reformism of the Second International and the pro-Moscow orientation of the Comintern.

In 1980, the POUM made its last electoral efforts, supporting Herri Batasuna in the Basque country and participating in the Left Bloc for National Liberation (BEAN - Unitat Popular) coalition in the Catalan parliamentary election, but the party was disintegrating.

British author George Orwell fought alongside members of the Independent Labour Party as part of POUM militias;[5] he recounted the experience in his book Homage to Catalonia.

The POUM is briefly mentioned in Joe Haldeman's science fiction novel The Forever War as a militia where "(y)ou obeyed an order only after it had been explained in detail; you could refuse if it didn't make sense.

Ian Fleming's From Russia with Love (Signet edition, p. 50) states that Rosa Klebb infiltrated the POUM and may have murdered Andrés Nin Pérez.