United Left (Spanish: Izquierda Unida [iθˈkjeɾðawˈniða], IU) is a federative political movement in Spain that was first organized as a coalition in 1986, bringing together several left-wing political organizations, grouped primarily around the Communist Party of Spain.

IU was founded as an electoral coalition of seven parties, but the Communist Party of Spain (PCE) is the only remaining integrated member of the IU at the national level. Despite that, IU brings together other regional parties, political organizations, and independents. It currently takes the form of a permanent federation of parties.

IU took part in the Unidas Podemos coalition and the corresponding parliamentary group in the Congreso de los Diputados between 2016 and 2023. Since January 2020, it participated for the first time in a national coalition government, with one minister. For the 2023 general election, IU took part in the Sumar platform.

Following the electoral failure of the PCE in the 1982 general election (going from 10% to 4% of the votes), PCE leaders believed that the PCE alone could no longer effectively challenge the electoral hegemony of the Spanish Socialist Workers' Party (PSOE) on the left. With this premise, the PCE began developing closer relations with other left-wing groups, with the vision of forming a broad left coalition. IU slowly improved its results, reaching 9% in 1989 (1,800,000 votes) and nearly 11% in 1996 (2,600,000 votes). The founding organizations were: Communist Party of Spain, Progressive Federation, Communist Party of the Peoples of Spain, PASOC, Carlist Party, Humanist Party, Unitarian Candidacy of Workers, and Republican Left.

In contrast to the PCE prior to the formation of IU, which pursued a more moderate political course, the new IU adopted a more radical strategy and ideology of confrontation against the PSOE. IU generally opposed cooperating with the PSOE, and identified it as a "right-wing party", no different from the People's Party (PP).

After achieving poor results in the 1999 local and European elections, IU decided to adopt a more conciliatory attitude towards the PSOE, and agreed to sign an electoral pact with the PSOE for the upcoming general election in 2000. They also adopted a universal policy in favor of cooperating with the PSOE at the local level.

IU currently has around 18,000 members, a decrease from 70,000 in 2012.

In 1986, the PCE promoted United Left (IU) and participated with other political forces in its foundation, first constituted as an electoral coalition and later as a federation of parties. At the 13th Congress of the PCE it was decided to transfer its sovereignty to the bodies of IU, with its secretary-general Gerardo Iglesias also becoming the first general coordinator of the new coalition.