1921 Persian coup d'tat, known in Iran as 3 Esfand 1299 coup d'tat (Persian: with the Solar Hijri date), refers to several major events in Persia in 1921, which eventually led to the establishment of the Pahlavi dynasty as the ruling house of the country in 1925.

The events began with a coup by the Persian Cossack Brigade headed by Reza Khan, and directed by the British, on 21 February 1921. With this coup Zia'eddin Tabatabaee took over power and became Prime Minister. The coup was largely bloodless and faced little resistance. With his expanded forces and the Cossack Brigade, Reza Khan launched successful military actions to eliminate separatist and dissident movements in Tabriz, Mashhad and the Jangalis in Gilan. The campaign against Simko and the Kurds was less successful and lasted well into 1922, though eventually concluding with Persian success.

Reza Shah Pahlavi (Persian: رضا شاه پهلوی; pronounced [ɾeˈzɒː ˈʃɒːh-e pæhlæˈviː]; originally Reza Khan (رضا خان); 15 March 1878 – 26 July 1944) was an Iranian military officer, politician (who served as minister of war and prime minister), and first Shah of the House of Pahlavi of the Imperial State of Iran and father of the last Shah of Iran. He reigned from 15 December 1925 until he was forced to abdicate by the Anglo-Soviet invasion of Iran on 16 September 1941. Reza Shah introduced many social, economic, and political reforms during his reign, ultimately laying the foundation of the modern Iranian state. Therefore, he is regarded as the founder of modern Iran.At the age of 14 he joined the Cossack Brigade, and also served in the army. In 1911, he was promoted to First Lieutenant, by 1912 he was elevated to the rank of Captain and by 1915 he became a Colonel. In February 1921, as leader of the entire Cossack Brigade based in Qazvin he marched towards Tehran and seized the capital. He forced the dissolution of the government and installed Zia ol Din Tabatabaee as the new Prime Minister. Reza Khan's first role in the new government was Commander-in-Chief of the Army and the Minister of War.

Two years after the coup, Seyyed Zia appointed Reza Pahlavi as Iran's prime minister, backed by the compliant national assembly of Iran. In 1925, Reza Pahlavi was appointed as the legal monarch of Iran by the decision of Iran's constituent assembly. The assembly deposed Ahmad Shah Qajar, the last Shah of the Qajar dynasty, and amended Iran's 1906 constitution to allow selection of Reza Pahlavi as the Shah of Iran. He founded the Pahlavi dynasty that lasted until overthrown in 1979 during the Iranian Revolution.In the spring of 1950, he was posthumously named as Reza Shah the Great (رضا شاه بزرگ) by Iran's National Consultative Assembly.His legacy remains controversial to this day. His defenders assert that he was an essential reunification modernization force for Iran (whose international prominence had sharply declined during Qajar rule), while his detractors assert that his reign was often despotic, with his failure to modernize Iran's large peasant population eventually sowing the seeds for the Iranian Revolution nearly four decades later, which ended 2,500 years of Persian monarchy. Moreover, his insistence on ethnic nationalism and cultural unitarism, along with forced detribalization and sedentarization, resulted in the suppression of several ethnic and social groups. Although he himself was of Iranian Mazanderani descent, his government carried out an extensive policy of Persianization trying to create a single, united and largely homogeneous nation, similar to Mustafa Kemal Atatürk's policy of Turkification in Turkey after the fall of the Ottoman Empire.