The Republican Left of Catalonia (Catalan: Esquerra Republicana de Catalunya [əsˈkɛrə rəpuβliˈkanə ðə kətəˈluɲə], ERC; generically branded as Esquerra Republicana) is a pro-Catalan independence, social democratic political party in the Spanish autonomous community of Catalonia, with a presence also in Valencia, the Balearic Islands and the French department of Pyrénées-Orientales (Northern Catalonia). It is also the main sponsor of the movement for independence from France and Spain in the territories known as Catalan Countries, focusing in recent years on the creation of a Catalan Republic in Catalonia-proper.

ERC members sit in the unicameral Catalan Parliament, which exercises devolved powers within Spain. The party also contests and wins elections for seats in both houses of the Spanish Cortes Generales (the national parliament), as well as the European Parliament, where it sits as a member of the European Free Alliance. In 2022, ERC had 9,047 members. It is headquartered in Barcelona. Currently, its president is Oriol Junqueras and its secretary-general is Elisenda Alamany.

ERC was founded almost 100 years ago, and has counted amongst its leaders Francesc Macià, Lluís Companys and Josep Tarradellas. ERC played an important role in Catalan and Spanish politics during the Second Republic, the Spanish Civil War, as part of the anti-Francoist resistance, and in Spain's transition to democracy. After a difficult period in the 1980s, it recovered a key electoral position during the 2000s, becoming a coalition partner in various Catalan governments. In 2021, an ERC member won the presidency of Catalonia for the first time since 1980, with the appointment of lawyer Pere Aragonès as President of the Generalitat de Catalunya (President of the Catalan Government).

After the fall of Primo de Rivera (1930), the Catalan left made great efforts to create a united front under the leadership of left-wing independentist leader Francesc Macià. The Republican Left of Catalonia was founded at the Conference of the Catalan Left (held in Sants, Barcelona, on 19 March 1931) as the union of the independentist Estat Català (Catalan State), led by Francesc Macià, the Catalan Republican Party, led by Lluís Companys, and the L'Opinió Group of Joan Lluhí i Vallescà.

The party did extremely well in the municipal elections of 12 April 1931. Two days later, on 14 April, few hours before the proclamation of the Spanish Republic in Madrid, Macià proclaimed in Barcelona the Catalan Republic within the Iberian Federation. This was not exactly what had been agreed in the Pact of San Sebastián, so three days later they negotiated with the Madrid government that Macià would become President of the Generalitat of Catalonia, an autonomous Catalan government within the recently founded Spanish Republic.

In September 1932, the Spanish Republican Cortes approved the Statute of Autonomy of Catalonia which, among other provisions, granted a Catalan Parliament with broad legislative powers, and it was elected on 20 November 1932. The Republican Left of Catalonia, in coalition with the Socialist Union of Catalonia and other minor left-wing parties, won a large majority of seats (67 of 85), while the previously hegemonic Regionalist League, representing a more conservative view of Catalan nationalism, came in second place but far behind ERC (17 from 85). From this strong position, the ERC through the Catalan Government sought to improve living conditions of the popular classes and the petite bourgeoisie, approving laws in areas such as in culture, health, education and civil law. The Catalan Government also progressed the Crop Contracts Law, which sought to protect tenant farmers and grant them access to the land they were cultivating; it was contested by the Regionalist League and provoked a legal dispute with the Spanish government. In October 1933, Joan Lluhí and other members of the l'Opinió Group, as well Josep Tarradellas, withdrew from the ERC due to disagreement with Macià over the distribution of powers between the Executive Council of Catalonia and the President of the Generalitat; they founded the Nationalist Republican Left Party (PNRE).

Francesc Macià died in office in December 1933 and Lluís Companys was elected by the Parliament of Catalonia as the new President of the Generalitat. On 6 October 1934, following the appointment of right-wing members of the Spanish Confederation of the Autonomous Right (CEDA) as ministers in the Government of the Spanish Republic, Companys unlawfully declared a Catalan State "within a Spanish Federal Republic" (which, of course, did not at that time exist). CEDA was considered close to fascism and, therefore, it was feared that the ministers' appointment was a first step toward suppressing the autonomy of Catalonia and taking complete power nationally. The nascent Catalan 'state' was quickly suppressed by the Spanish Army, and members of the Catalan government were arrested. Party leaders (including Companys himself) and Catalan officials were tried by the Supreme Court of the Republic, receiving jail sentences, while the Statute of Autonomy of Catalonia was suspended (until February 1936).

In 1936, at the dawn of the Spanish Civil War, ERC became part of the Popular Front to contest that year's election. Esquerra became the leading force of the Popular Front, (called Front d'Esquerres, "Left Front" in Catalan) in Catalonia, which it won 41 from 54 Catalan seats, 21 of them belonging to ERC. The new left-wing Spanish government pardoned Companys and the members of the Catalan government, restoring the self-government. In June 1936, Estat Català split from ERC, while the PNRE rejoined it.