The Popular Front (Spanish: Frente Popular) in Spain's Second Republic was an electoral alliance and pact signed in January 1936 by various left-wing political organizations, instigated by Manuel Azaña for the purpose of contesting that year's election. In Catalonia and today's Valencian Community the name of the coalition was Front d'Esquerres (in Catalan, meaning Front of the Left).The Popular Front included the Spanish Socialist Workers' Party (PSOE), Communist Party of Spain (PCE), and the republicans: Republican Left (IR), (led by Azaña) and Republican Union (UR), led by Diego Martínez Barrio. This pact was supported by Galician (PG) and Catalan nationalists (ERC), the POUM, socialist union Workers' General Union (UGT), and the anarchist trade union, the Confederación Nacional del Trabajo (CNT). Many anarchists who would later fight alongside Popular Front forces during the Spanish Civil War did not support them in the election, urging abstention instead.

The Comintern had decided in 1935 that, in response to the growth of Fascism, popular fronts allying Communist parties with other anti-Fascist parties including Socialist and even bourgeois parties were advisable. In Spain, it was a coalition between leftist republicans and workers' organizations to defend social reforms of the first government (1931–1933) of the Second Spanish Republic, and liberate the prisoners, political prisoners according with the front propaganda, held since the Asturian October Revolution (1934).

The Popular Front defeated the National Front (a collection of right-wing parties) and won the 1936 election, forming the new Spanish Government. Manuel Azaña was elected President of the Republic on May 1936, but the PSOE did not join the government because of the opposition of Francisco Largo Caballero.

In July 1936, conservative/monarchist generals instigated a coup d'état which started the Spanish Civil War (1936–1939). The Government dissolved the Spanish Republican Army in the loyal territory and brought weapons to armed groups organized by the unions (UGT and CNT) and workers' parties (PSOE, PCE, POUM) that had initial success in defeating the Francoist forces in Madrid, Barcelona, Bilbao and Valencia. In October the same year, the Spanish Republican Army was reorganized. After a protracted war of attrition Franco would defeat the Republican forces and rule Spain as a dictatorship until he died in 1975.