Rezā Shāh, also known as Rezā Shāh Kabir (Reza Shah the Great), or Rezā Shāh Pahlavi (March 16, 1878 – July 26, 1944), was the Shah of the Imperial State of Iran from December 15, 1925 until he was forced to abdicate by the Anglo-Soviet invasion of Iran in September 16, 1941.

In 1925, Reza Shah overthrew Ahmad Shah Qajar, the last Shah of the Qajar dynasty, and founded the Pahlavi Dynasty. He established an authoritarian government that valued nationalism, militarism, secularism and anti-communism combined with strict censorship and state propaganda. Known as quite intelligent despite his lack of formal education, Reza Shah introduced many socio-economic reforms, reorganizing the army, government administration, and finances. To his supporters his reign brought "law and order, discipline, central authority, and modern amenities - schools, trains, buses, radios, cinemas, and telephones". However, his attempts of modernisation have been criticised for being "too fast" and "superficial", and his reign a time of "oppression, corruption, taxation, lack of authenticity" with "security typical of police states." For many Iranian nationalists he is considered the father of modern Iran.

In the early stages of his life, Reza Shah was known as Reza Savad-Koohi, or Reza Khan because of his birth place. Later on, when he gained territory with his own army, he entitled himself Reza Khan, and later as Reza Khan Mirpanj, his full military title at the time. Upon becoming minister of war, he was known as Reza Khan Sardar Sepah, which in Persian roughly means Reza Khan, head of the armed forces. Upon securing his position as the Shah of Persia, he chose the surname Pahlavi (surnames did not exist in Persia before this date, and were introduced as one of the modernization measures during his reign).

Reza Pahlavi was born in the village of Alasht in Savad Kooh county, Mazandaran in 1878. It is believed that Reza Shah Pahlavi's grandmother was a Georgian (from Mazandaran). When Reza was sixteen years old, he joined the Persian Cossack Brigade, in which, years later, he would rise to the rank of Brigadier. He also served in the Iranian Army, where he gained the rank of gunnery sergeant under Qajar Prince Abdol Hossein Mirza Farmanfarma's command. He rose through the ranks, eventually holding a commission as a Brigadier General in the Persian Cossack Brigade. He was the last and only Iranian commander of the Persian Cossack Brigade. He was also one of the last individuals to become an officer of the Neshan-e Aqdas prior to the collapse of the Qajar dynasty in 1925. In late 1920 the Soviet Socialist Republic in Rasht was preparing to march on Tehran with "a guerrilla force of 1500 Jangalis, Kurds, Armenians, and Azerbaijanis", reinforced by the Soviet Red Army. This fact, along with various other disorders, mutinies and unrest in the country created "an acute political crisis in the capital." On February 21, 1921, Reza Khan staged a coup d'état together with Seyyed Zia'eddin Tabatabaee, to get control over a country which had practically no functioning central government at the time. Commanding a Russian-trained Cossack Brigade, Reza Khan marched his troops from Qazvin, 150 kilometres to the west of Tehran, and seized key parts of the capital city almost without opposition and forced the government to resign. With the success of the coup, Tabatabaee became the Prime Minister of Iran. Reza Khan's first role in the new government was as commander of the army, which, in April 1921, he combined with the post of Minister of War. At the same time, he took the title Reza Khan Sardar Sepah.

While Reza Khan and his Cossack brigade were securing Tehran, the Persian envoy was in Moscow negotiating a treaty with the Bolsheviks for the removal of Soviet troops from Persia. Known as the 1921 Treaty of Friendship, its Article IV allowed the Soviets to invade and occupy Persia should they believe foreign troops were using Persia as a staging area for an invasion of Soviet territory. As Soviets interpreted the Treaty, they could invade should events in Persia prove threatening to Soviet national security. 

The coup d'état of 1921 and the emergence of Reza Khan were assisted by the British government which wished to halt the Bolsheviks' penetration of Iran particularly because of the threat it posed to the British colonial possession of India. It is thought that British provided "ammunition, supplies and pay" for Reza's troops. Before the coup, the commander of the British Forces in Iran, General Edmond Ironside, gave a situation report to the British War Office opining that a capable Persian officer must command the Cossacks and this "would solve many difficulties and enable us to depart in peace and honour."

In 1921 there were a number of revolts against the coup. In June 1920, a soviet socialist republic had been established in Gilan by Mīrzā Kūchak Khān, as the prime minister. Kurds of Khorasan also revolted in the same year. On October 26, 1923, Reza had seized control of Iran and forced the young Ahmad Shah Qajar to exile in Europe. As the Prime Minister, Reza Khan wanted to secure his power in opposition to any potential restoration of Qajar house. He now machinated for a republic and his military junta started a massive propaganda campaign for establishment of a republic. However, the idea of a republic was fiercely opposed by the powerful clergymen, and the feudal landlords. Some leaders of the National Assembly of Iran, known as the Majlis, particularly Hassan Modarres and the young Dr. Mohammed Mossadegh forcefully opposed Reza Khan’s plan to consolidate his autocracy. His supremacy was imposed by 1925 with the subjugation of all tribal insurrections and nationalists’ unrest. He maneuvered against Qajar dynasty and in October forced the parliament to depose the young King. He assured the landlords and the conservative clergy that he would defend Islamic law and would not undertake any radical reforms. The Majlis, convening as a constituent assembly on December 12, 1925, declared him the Shah. Three days later, on December 15, 1925, he took his imperial oath and thus became the first Shah of the Pahlavi dynasty. It was not until April 25, 1926 that Reza Shah would receive his coronation and first place the Imperial Crown on his head. At the same ceremony his son, Mohammad Reza Pahlavi, was proclaimed the Crown Prince of Persia – to rule after his father.

During Reza Shah's sixteen years of rule, major developments, such as large road construction projects and the Trans-Iranian Railway were built, modern education was introduced and the University of Tehran was established. The government sponsored European educations for many Iranian students. The number of modern industrial plants, increased 17 fold under Reza Shah, (excluding oil installations), the number of miles of highway increased from 2000 to 14,000. One area of modernization his regime failed in was public health. Along with the modernization of the nation, Reza Shah was the ruler during the time of the Women's Awakening (1936-1941). This movement sought the elimination of the Islamic veil from Iranian society. Supporters held that the veil impeded physical exercise and the ability of women to enter society and contribute to the progress of the nation. This move met opposition from the religious establishment. The unveiling issue and the Women's Awakening are linked to the Marriage Law of 1931 and the Second Congress of Eastern Women in Tehran in 1932. Reza Shah was the first Iranian Monarch after 1400 years who paid respect to the Jews by praying in the synagogue when visiting the Jewish community of Isfahan; an act that boosted the self-esteem of the Iranian Jews and made Reza Shah their second most respected Iranian leader after Cyrus the Great. Reza Shah's reforms opened new occupations to Jews and allowed them to leave the ghetto.

He forbade photographing aspects of Iran he considered backwards such as camels, and he banned Islamic dress and chadors in favour of Western dress. Women who resisted this compulsory unveiling had their veils forcibly removed. He dealt harshly with opposition: troops were sent to massacre protesters at mosques and nomads who refused to settle; newspapers were closed and liberals imprisoned. He also used his power to vastly increase his fortune, becoming the biggest landowner in Iran, proprietor of nearly three thousand villages, as well as many factories and enterprises. Reza Shah would discredit and eliminate his ministers whom he considered as his rivals. His minister of Imperial Court, Abdolhossein Teymourtash, was accused and convicted of corruption, bribery, misuse of foreign currency regulations, and plans to overthrow the Shah. He was removed as the minister of court in 1932 and was murdered while in prison in September 1933. His minister of finance, Prince Firuz Mirza Nosrat-daula who played an important role in the first three years of his reign was convicted on similar charges in May 1930 and was murdered in prison in January 1938. Ali-Akbar Davar, his minister of justice, was suspected of similar charges and committed suicide in February 1937.

As his reign became more secure, Reza Shah clashed with Iran's clergy and devout Muslims on many issues. In March 1928 he violated the sanctuary of Qom's Fatima al-Masumeh Shrine and beat a cleric who had angrily admonished Reza Shah's wife for temporarily exposing her face a day earlier while on pilgrimage to Qom. In December of that year he instituted a law requiring everyone to wear Western clothes. This angered devout Muslims because it included a hat with a brim which prevented the devout from touching their foreheads on the ground during salah as required by Islamic law. The Shah also encouraged women to discard hijab. He announced that female teachers could no longer come to school with head coverings. The devout were also angered by policies which allowed mixing of the sexes. Women were allowed to study in the colleges of law and medicine, and in 1934 a law set heavy fines for cinemas, restaurants, and hotels that did not open doors to both sexes. Doctors were permitted to dissect human bodies. He restricted public mourning observances to one day and required mosques to use chairs during these observances instead of the traditional sitting on the floors of mosques. By the mid-1930s, Reza Shah's rule had caused intense dissatisfaction within the Shi'a clergy throughout Iran. In 1935 a backlash erupted in the Mashed shrine. Responding to a cleric who denounced the Shah's heretical innovations, corruption and heavy consumer taxes, many bazaaris and villagers took refuge in the shrine, chanting slogans such as `The Shah is a new Yezid.` For four full days local police and army refused to violate the shrine. The standoff was ended when troops from Azerbaijan arrived and broke into the shrine, killing dozens and injuring hundreds, and marking a final rupture between Shi'ite clergy and the Shah. The shah intensified his controversial changes following the incident, banning the chador and ordering all citizens - rich and poor - to bring their wives to public functions without head coverings.

Reza Shah initiated changes in foreign affairs as well. Despite the support initially given to him by the British, the Shah worked to balance British influence with other foreigners and generally to diminish foreign influence in Iran. In 1931, he refused to allow Imperial Airways to fly in Persian airspace, instead giving the concession to German-owned Lufthansa Airlines. The next year he surprised the British by unilaterally canceling the oil concession awarded William Knox D'Arcy (then called Anglo-Persian Oil Company), which was slated to expire in 1961. The concession granted Persia 16% of the net profits from APOC oil operations. The Shah wanted 21%. Following a brief challenge by the British before the League of Nations, the British acquiesced. He previously hired American consultants to develop and implement Western-styled financial and administrative systems. Included among them was U.S. economist Dr. Arthur Millspaugh, who acted as the nation's Finance Minister. Reza Shah also purchased ships from Italy and hired Italians to teach his troops the intricacies of naval warfare. He also began bringing in hundreds of German technicians and advisors for various projects. Mindful of Persia's long period of subservience to British and Russian authority, Reza Shah was careful to avoid giving any one foreign nation too much control. He also insisted that foreign advisors be employed by the Persian government so that they would not be answerable to foreign powers. This was based upon his experience with Anglo-Persian which was owned and operated by the British government. In his campaign against foreign influence he annulled the 19th century capitulations to Europeans in 1928. Under these, Europeans in Iran had enjoyed the privilege of being subject to their own consular courts rather than to the Iranian judiciary. The right to print money was moved from the British Imperial Bank to his National Bank of Iran (Bank-i Melli Iran), as was the administration of the telegraph system from the Indo-European Telegraph Company to the Iranian government, in addition to the collection of customs by Belgian officials. He eventually fired Millspaugh, and prohibited foreigners from administering schools, owning land or traveling in the provinces without police permission.

On 21 March 1935, he 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 for a country identified as Iran in the Persian language. It has however contributed more to the Iranian people than others, particularly its language. Opponents claimed that this act brought cultural damage to the country and separated Iran from its past in the West. The name Iran means “Land of the Aryans”.

Tired of opportunistic policies of both Britain and the Soviet Union, the shah circumscribed contacts with foreign embassies. Relations with the Soviet Union had already deteriorated because of that country's commercial policies, which in the 1920s and 1930s adversely affected Iran. In 1932 the shah offended Britain by canceling the agreement under which the Anglo-Persian Oil Company produced and exported Iran's oil. Although a new and improved agreement was eventually signed, it did not satisfy Iran's demands and left bad feelings on both sides. To counterbalance British and Soviet influence, Reza Shah encouraged German commercial enterprise in Iran. On the eve of World War II, Germany was Iran's largest trading partner. The Germans agreed to sell him the steel factory he coveted and considered a sine qua non of progress and modernity. Nevertheless, according to the British embassy reports from Tehran in 1940, the total number of German citizens in Iran - from technicians to spies - was no more than a thousand.

His foreign policy, which had consisted essentially of playing the Soviet Union off against Great Britain, failed when those two powers joined in 1941 to fight the Germans. To supply the Soviet forces with war material through Iran, the two allies jointly occupied the country in August 1941 by a massive air, land, and naval assault subsequently forcing Reza Shah to abdicate in favour of his son. The Shah received with disbelief, as a personal humiliation and defeat, news that fifteen Iranian divisions had surrendered without much resistance. Some of his troops dispersed and went home, while others were locked up in their barracks by the Allies. The invasion was allegedly in fear that Reza Shah was about to align his petroleum-rich country with Nazi Germany during the war. However, Reza Shah's earlier Declaration of Neutrality and refusal to allow Iranian territory to be used to train, supply, and act as a transport corridor to ship arms to Russia for its war effort against Germany, was the strongest motive for the allied invasion of Iran. Because of its importance in the allied victory, Iran was subsequently called "The Bridge of Victory" by Winston Churchill.

The Shah's son, Mohammad Reza Pahlavi, officially replaced his father on the throne on September 16, 1941. Reza Shah was soon forced into exile by the British forces to British territories, first to Mauritius, then to Durban thence Johannesburg, South Africa, where he died on July 26, 1944, of heart ailment from which he had been complaining for many years. He was sixty-six years old at the time of his death. After his passing, his body was carried to Egypt, where it was embalmed and kept at the royal Al Rifa'i Mosque in Cairo (poignantly, the future burial place of his son, the exiled Mohammad Reza Pahlavi). Many years later, the remains were flown back to Iran, where the embalming was removed (Islamic laws do not allow for embalming of the dead), and buried in a beautifully designed and decorated mausoleum built in his honor at the Shia shrine town of Ray/Shah-Abdol-Azim, in the southern suburbs of the capital, Tehran. The Iranian parliament (Majlis) later designated the title "the Great " to be added to his name.

Following the Iranian Revolution in 1979, Reza Shah's mausoleum was destroyed under the direction of Ayatollah Sadeq Khalkhali, which was sanctioned by Ayatollah Ruhollah Khomeini.