Tehran is the capital and largest city of Iran. It is also the capital of Tehran province and the administrative center for Tehran County and its Central District. With a population of around 9 million in the city, and 16.8 million in the metropolitan area, Tehran is the most populous city in Iran and West Asia, the largest metropolitan area in the Middle East and the 24th-most-populous metropolitan area in the world. Greater Tehran includes several municipalities, including Eslamshahr, Shahriar, Qods, Malard, Golestan, Varamin, Pakdasht, Qarchak, Nasimshahr, Parand, Pardis, Andisheh, Fardis, Tajrish, and Rey.
In classical antiquity, part of the territory of present-day Tehran was occupied by Rhages, a prominent Iranian city that was not destroyed in the medieval Arab and Mongol invasions. Rey is absorbed into the metropolitan area of Greater Tehran. Tehran was first chosen as the capital of Iran in 1786 by Agha Mohammad Khan of the Qajar dynasty, because of its proximity to Iran's territories in the Caucasus—which were contested in the Russo-Iranian Wars—and to avoid the vying factions of prior ruling Iranian dynasties; the capital of Iran had been moved several times throughout its long history, with Tehran becoming the 51st. Under Reza Shah, Tehran witnessed Iran's first institute of higher learning, bank, railway line, and museum. Large-scale construction works began in the 1920s, and Tehran became a destination for mass migrations from all over Iran, especially in the 20th century. Tehran was at the center of the Iranian Revolution, and experienced major bombing damage during the Iran–Iraq War, the Twelve-Day War, and the 2026 Iran war.
Tehran is home to many historical sites, including the World Heritage Site Golestan Palace of the Qajar dynasty and the Masoudieh, Sa'dabad, Niavaran and Marmar palace complexes of both the Qajar and Pahlavi dynasties. Landmarks include the Azadi Tower, a memorial built in 1971 to mark the 2,500th anniversary of the Persian Empire; the Milad Tower, the world's sixth-tallest self-supporting tower, completed in 2007; and the Tabiat Bridge, completed in 2014.
Tehran has been described as a cultural "melting pot", hosting more Azerbaijanis than any other city in the world. Tehran is served by Imam Khomeini International Airport, alongside the domestic Mehrabad Airport, a central railway station, Tehran Metro, the Tehran Bus Rapid Transit system, trolleybuses, and a large network of highways.
Because of air pollution, earthquakes and water scarcity, there have been plans to relocate the capital to another area, although none have been approved. A 2016 survey of 230 cities across the globe by Mercer ranked Tehran 203rd for quality of life. According to the Global Destinations Cities Index in 2016, Tehran was among the top ten fastest growing tourism destinations. In 2016, the Tehran City Council declared 6 October "Tehran Day", celebrating the date in 1907 when the city officially became the capital of Iran.
Various theories on the origin of the name Tehran have been put forward. Iranian linguist Zana Kamrani, in an article "Shemiran-Tehran", suggests that Tehran, and Kehran mean "the warm place", and "Shemiran" means "the cool place". He lists cities with the same base and suffix and studied the components of the word in ancient Iranian languages, and came to the conclusion that Tehran and Kehran meant the same thing in different Iranian language families, as the constant "t" and "k" are close to each other in such languages. He also provided evidence that cities named "Shemiran" were colder than those named "Tehran" or "Kehran". He considered other theories not consider the ancient history of Iranian languages, such as "Tirgan" theory and "Tahran" theory folk etymology.
The official City of Tehran website says that "Tehran" comes from the Persian words "Tah" meaning "end", or "bottom", and "Ran" meaning "[mountain] slope"; the bottom of the mountain (ته کوه), referring to Tehran's position at the foot of the Alborz mountains.
In English, it is also spelt "Teheran", with both variants being used in books since at least 1800, and "Teheran" being the dominant form from after WWII until shortly before the Islamic Revolution.
Archaeological remains from the ancient city of Ray suggest that settlement in Tehran dates back over 6,000 years.
Tehran is in the historical Media region (Old Persian: 𐎶𐎠𐎭 Māda) in northwestern Iran. By the time of the Median Empire, part of present-day Tehran was a suburb of the prominent Median city of Rhages (Old Persian: 𐎼𐎥𐎠 Ragā). In the Avesta's Videvdat (i, 15), Rhages is mentioned as the 12th sacred place created by Ohrmazd. In Old Persian inscriptions, Rhages appears as a province (Bistun 2, 10–18). From Rhages, Darius I sent reinforcements to his father Hystaspes, who was putting down a rebellion in Parthia (Bistun 3, 1–10). Some Middle Persian texts give Rhages as the birthplace of Zoroaster, although modern historians generally place the birth of Zoroaster in Khorasan province.
Mount Damavand, the highest peak of Iran, which is located near Tehran, is an important location in Ferdowsi's Šāhnāme, an Iranian epic poem based on the ancient legends of Iran. It appears in the epics as the homeland of the protoplast Keyumars, the birthplace of King Manuchehr, the place where King Fereydun bound the dragon fiend Zahhak (Bivarasp), and the place where Arash shot his arrow.
In 641, during the reign of the Sasanian Empire, Yazdgerd III issued his last appeal to the nation from Rhages, before fleeing to Khorasan. Rhages was dominated by the Parthian House of Mihran, and Siyavakhsh—the son of Mehran, the son of Bahram Chobin—who resisted the seventh-century Muslim invasion of Iran. Because of this resistance, when the Arabs captured Rhages, they ordered the town destroyed and rebuilt anew by the aristocrat Farrukhzad.
In the ninth century, Tehran was a well-known village, but less so than Rhages, flourishing nearby. Rhages was described in detail by tenth-century Muslim geographers. Despite the interest that Arabian Baghdad displayed in Rhages, the number of Arabs in the city remained insignificant and the population mainly consisted of Iranians of all classes.
The Oghuz Turks invaded Rhages in 1035 and again in 1042, but the city was recovered under the Seljuks and the Khwarezmians. Medieval writer Najm od Din Razi declared the population of Rhages about 500,000 before the Mongol invasion. In the 13th century, the Mongols invaded Rhages, laid the city to ruins, and massacred many of its inhabitants. Others escaped to Tehran. In July 1404, Castilian ambassador Ruy González de Clavijo visited Tehran on a journey to Samarkand, the capital of Turco-Mongol conqueror Timur, the ruler of Iran at the time. He described it in his diary as an unwalled region.
Italian traveler Pietro della Valle passed through Tehran overnight in 1618, and in his memoirs called the city Taheran. English traveler Thomas Herbert entered Tehran in 1627, and mentioned it as Tyroan. Herbert states that the city had about 3,000 houses.
In the early 18th century, Karim Khan of the Zand dynasty ordered a palace and a government office built in Tehran, possibly to declare the city his capital; but he later moved his government to Shiraz. Eventually, Qajar king Agha Mohammad Khan chose Tehran as the capital of Iran in 1786. Khan's choice of his capital was based on a similar concern for the control of both northern and southern Iran. He was aware of the loyalties of the inhabitants of former capitals Isfahan and Shiraz to the Safavid and Zand dynasties respectively, and was wary of the power of the local notables in these cities. Thus, he probably viewed Tehran's lack of a substantial urban structure as a blessing, because it minimized the chances of resistance to his rule by the notables and by the general public. He had to remain within close reach of Azerbaijan and Iran's integral northern and southern Caucasian territories—at that time not yet irrevocably lost per the treaties of Golestan and Turkmenchay to the neighboring Russian Empire—which would follow in the course of the 19th century.