The Imperial State of Iran, officially known as the Imperial State of Persia until 1935 and commonly referred to as Pahlavi Iran, was the Iranian state under the rule of the Pahlavi dynasty. The Pahlavi dynasty was created in 1925 and lasted until 1979 when it was ousted as part of the Iranian Revolution, which ended the Iranian monarchy and established the Islamic Republic of Iran.

The Pahlavis came to power in 1925 with the ascension to the throne of Reza Shah, a former brigadier-general of the Persian Cossack Brigade, and the overthrow of Ahmad Shah Qajar, the last Iranian ruler under the Qajar dynasty. Iran's Majlis, convening as a constituent assembly on 12 December 1925, deposed the young Ahmad Shah Qajar and declared Reza Shah as the new shah of the Imperial State of Persia. In 1935, Reza Shah asked foreign delegates to use the endonym Iran instead of the exonym Persia when addressing the country in formal correspondence. Reza Shah declared Iran neutral during the Second World War. Nonetheless, Iran was occupied by British and Soviet forces following the Anglo-Soviet invasion of Iran. Subsequently Reza Shah was forced to abdicate.

After Reza Shah's forced abdication, he was succeeded by his son Mohammad Reza Pahlavi, who became the last Shah of Iran. By 1953, Mohammad Reza Shah's rule became more autocratic and firmly aligned with the Western Bloc during the Cold War in the aftermath of the 1953 Iranian coup d'état, which was engineered by the United Kingdom and the United States. In correspondence with this reorientation of Iran's foreign policy, the country became an ally of the United States in order to act as a bulwark against Soviet ideological expansionism, and this gave the Shah the political capital to enact a hitherto unprecedented socio-economic program that would transform all aspects of Iranian life through the White Revolution. Consequently, Iran experienced prodigious success in all indicators, including literacy, health, and standard of living. By 1978, the Shah faced growing public discontent that culminated into a full-fledged popular revolutionary movement led by religious cleric Ruhollah Khomeini. Mohammad Reza Shah went into exile with his family in January 1979, sparking a series of events that quickly led to the end of monarchy and the establishment of the Islamic Republic on 31 March 1979. Following Mohammed Reza Pahlavi's death in 1980, his son Reza Pahlavi now leads the exiled family throne.

In 1925, Reza Khan, a former brigadier-general of the Persian Cossack Brigade, deposed the Qajar dynasty and declared himself king (shah), adopting the dynastic name of Pahlavi, which recalls the Middle Persian language of the Sasanian Empire. He had chosen the last name Pahlavi for himself in November 1919. By the mid-1930s, Reza Shah's strong secular rule had the support of the middle and upper-middle class, but caused opposition among some groups, particularly the clergy. In 1935, Rezā Shāh issued a decree asking foreign delegates to use the term Iran in formal correspondence, in accordance with the fact that "Persia" was a term used by Western people for the country called "Iran" in Persian. His successor, Mohammad Reza Pahlavi, announced in 1959 that both Persia and Iran were acceptable and could be used interchangeably.

Reza Shah tried to avoid involvement with the UK and the Soviet Union. Though many of his development projects required foreign technical expertise, he avoided awarding contracts to British and Soviet companies because of dissatisfaction during the Qajar Dynasty between Iran, the UK, and the Soviets. Although the UK, through its ownership of the Anglo-Iranian Oil Company, controlled all of Iran's oil resources, Reza Shah preferred to obtain technical assistance from Germany, France, Italy and other European countries. This created problems for Iran after 1939, when Germany and Britain became enemies in World War II. Reza Shah proclaimed Iran as a neutral country, but Britain insisted that German engineers and technicians in Iran were spies with missions to sabotage British oil facilities in southwestern Iran. Britain demanded that Iran expel all German citizens, but Reza Shah refused, claiming this would adversely affect his development projects.

Iran claimed to be a neutral country during the opening years of World War II. In April 1941, the war reached Iran's borders when Rashid Ali, with assistance from Germany and Italy, launched the 1941 Iraqi coup d'état, sparking the Anglo-Iraqi War of May 1941. Germany and Italy quickly sent the pro-Axis forces in Iraq military aid from Syria but during the period from May to July the British and their allies defeated the pro-Axis forces in Iraq and later Syria and Lebanon.

In June 1941, Nazi Germany broke the Molotov–Ribbentrop Pact and invaded the Soviet Union, Iran's northern neighbor. The Soviets quickly allied themselves with the Allied countries, and in July and August 1941 the British demanded that the Iranian government expel all Germans from Iran. Reza Shah refused to expel the Germans, and on 25 August the British and Soviets launched a surprise invasion, and Reza Shah's government quickly surrendered after less than a week of fighting. The invasion's strategic purpose was to secure a supply line to the Soviet Union (later named the Persian Corridor), secure the oil fields and Abadan Refinery (of the UK-owned Anglo-Iranian Oil Company), and limit German influence in Iran. Following the invasion, on 16 September 1941 Reza Shah abdicated and was replaced by Mohammad Reza Pahlavi, his 21-year-old son.

During the rest of World War II, Iran became a major conduit for British and American aid to the Soviet Union and an avenue through which over 120,000 Polish refugees and Polish Armed Forces fled the Axis advance. At the 1943 Tehran Conference, the Allied "Big Three"—Joseph Stalin, Franklin D. Roosevelt, and Winston Churchill—issued the Tehran Declaration to guarantee the post-war independence and boundaries of Iran.