Socialism in Iran, also called Iranian socialism, is a political ideology that has shaped Iranian politics since the early 20th century. The socialist tradition in Iran emerged during the Constitutional Revolution of 1905–1911, when Iranian intellectuals and workers came into contact with European and Russian radical thought, and it developed through a succession of parties, movements, and revolutionary organisations over the following decades.

The earliest socialist organisations in Iran were founded by Iranian emigrants in Transcaucasia who maintained close ties with the Russian Social Democratic Labour Party. During the constitutional era, several competing socialist and social-democratic parties competed in the nascent Iranian parliament. After the First World War and the Bolshevik Revolution, a more explicitly communist current emerged, culminating in the short-lived Gilan Soviet Republic (1920–1921) and the formation of the Communist Party of Iran. Under Reza Shah's authoritarian rule in the 1920s–1940s, independent political parties were suppressed, and organised socialist activity was largely driven underground or abroad.

Following Reza Shah's forced abdication in 1941, a new era of political pluralism briefly flourished. The Tudeh Party of Iran, founded in 1941, became the largest communist organisation in the country and at its height commanded significant popular support. At the same time, a broader nationalist-socialist current coalesced around Mohammad Mosaddegh and the National Front, which led the successful campaign to nationalise the Iranian oil industry and briefly governed Iran between 1951 and 1953. The 1953 Iranian coup d'état, engineered with British and American support, toppled Mosaddegh and inaugurated a period of renewed repression that scattered the Iranian left.

In the 1960s and 1970s, a new generation of socialist activists rejected both the Tudeh Party's alignment with the Soviet Union and the National Front's parliamentary approach, turning instead to Marxist-Leninist guerrilla strategy. The principal organisation of this tendency was the Organization of Iranian People's Fedai Guerrillas, founded in 1963. Although many on the left initially supported the Iranian Revolution of 1979, the new Islamic Republic quickly moved against socialist and communist groups, executing hundreds of activists in the early 1980s and forcing the surviving organisations into exile. Since then, Iranian socialist parties have continued to operate primarily from abroad, advocating for a secular democratic republic.

Socialist ideas first entered Iran through the networks of Iranian workers and intellectuals in Tsarist Transcaucasia, particularly in Baku, who were in close contact with Russian and Azerbaijani social democrats. The Social Democratic Party, established around 1904–1905, was Iran's first socialist organisation and maintained close links with the Russian Social Democratic Labour Party. It combined European socialist principles with nationalist and liberal ideas, and played a significant role in the Constitutional Revolution of 1905–1911 by mobilising popular support for a constitutional monarchy and the curtailment of royal and clerical power.

During the constitutional era, Iran's new parliament (the Majlis) saw the formation of several competing parties with varying degrees of socialist orientation. The Democrat Party, founded in 1909, was the more radical of the two main parliamentary groupings, advocating the separation of religion and state and drawing its membership largely from middle-class intellectuals with connections to the Baku-based Social Democratic Party. Its rival, the Moderate Socialists Party, represented a more conservative and clerical-influenced current. Other organisations from this period included the Unified Socialist Party, the Union and Progress Party, and the Socialist Revolutionary Party, the last of which was based in Baku and was among the most significant organisations founded by Iranian emigrants in Transcaucasia.

The Bolshevik Revolution of 1917 profoundly influenced the Iranian left. The Justice Party, founded in 1917 by former members of the Social Democratic Party, aligned itself with the Bolsheviks and participated in the Communist International, renaming itself the Communist Party of Iran in 1920. That same year, a short-lived Soviet Republic was proclaimed in Gilan under the Jangali movement leader Mirza Kuchik Khan, with communist support. The experiment ended in defeat in 1921, and the Young Communist League of Persia, which had been active in the Gilan uprising, was subsequently suppressed.

Following Reza Khan's rise to power—first as prime minister and then as shah in 1925—Iran's nascent socialist and labour movements were systematically dismantled. The Socialist Party, which had been one of four political groups courted by Reza Khan as he consolidated power, refused to endorse his claim to the throne on republican grounds. In the years following his coronation, the party was dissolved, its leaders forced into retirement, and its offices and affiliated institutions attacked by mobs organised with government support. Most other independent political parties, socialist or otherwise, met a similar fate. In 1937, a group of 53 intellectuals and activists—including several who would later help found the Tudeh Party—were imprisoned on charges of communist conspiracy, illustrating the Shah's thoroughgoing repression of the left.