Iran, officially the Islamic Republic of Iran (IRI) and also known as Persia, is a country in West Asia. It borders Iraq to the west, Turkey, Azerbaijan, and Armenia to the northwest, the Caspian Sea to the north, Turkmenistan to the northeast, Afghanistan to the east, Pakistan to the southeast, and the Gulf of Oman and the Persian Gulf to the south. With a population of 92 million, Iran ranks 17th globally in both geographic size and population and is the sixth-largest country in Asia. Iran is divided into five regions with 31 provinces. Tehran is the nation's capital, largest city, and financial center.

Iran is home to one of the world's oldest continuous major civilizations. A large part of Iran was first unified as a nation by the Medes under Cyaxares in the 7th century BCE and reached its territorial height in the 6th century BCE, when Cyrus the Great founded the Achaemenid Empire. Alexander the Great conquered the empire in the 4th century BCE. An Iranian rebellion in the 3rd century BCE established the Parthian Empire, which later liberated the country. In the 3rd century CE, the Parthians were succeeded by the Sasanian Empire, who oversaw a golden age in the history of Iranian civilization. During this period, ancient Iran saw some of the earliest developments of writing, agriculture, urbanization, religion, and administration. Once a center for Zoroastrianism, the 7th century CE Muslim conquest brought about the Islamization of Iran. Innovations in literature, philosophy, mathematics, medicine, astronomy and art were renewed during the Islamic Golden Age and Iranian Intermezzo, a period during which Iranian Muslim dynasties ended Arab rule and revived the Persian language. This era was followed by Seljuk and Khwarazmian rule, Mongol conquests and the Timurid Renaissance from the 11th to 14th centuries.

In the 16th century, the native Safavid dynasty re-established a unified Iranian state with Twelver Shia Islam as the official religion, laying the framework for the modern state of Iran. During the Afsharid Empire in the 18th century, Iran was a leading world power, but it lost this status after the Qajars took power in the 1790s. The early 20th century saw the Persian Constitutional Revolution and the establishment of the Pahlavi dynasty by Reza Shah, who ousted the last Qajar Shah in 1925. Following the Anglo-Soviet invasion of Iran in 1941, his son Mohammad Reza Pahlavi has rise to power. Attempts by Mohammad Mosaddegh to nationalize the oil industry led to the Anglo-American coup in 1953. The Iranian Revolution in 1979 overthrew the monarchy, and the Islamic Republic of Iran was established by Ruhollah Khomeini, the country's first supreme leader. In 1980, Iraq invaded Iran, sparking the eight-year-long Iran–Iraq War, which ended in a stalemate. Iran has since been involved in proxy wars with Israel, Saudi Arabia, and Turkey; in 2025, Israeli strikes on Iran escalated tensions into the Iran–Israel war.

Iran is an Islamic theocracy governed by elected and unelected institutions, with ultimate authority vested in the supreme leader. While Iran holds elections, key offices—including the head of state and military—are not subject to public vote. The Iranian government is authoritarian and has been widely criticized for its poor human rights record, including restrictions on freedom of assembly, expression, and the press, as well as its treatment of women, ethnic minorities, and political dissidents. International observers have raised concerns over the fairness of its electoral processes, especially the vetting of candidates by unelected bodies such as the Guardian Council. Iran maintains a centrally planned economy with significant state ownership in key sectors, though private enterprise exists alongside this. Iran is a middle power, due to its large reserves of fossil fuels (including the world's second largest natural gas supply and third largest proven oil reserves), its geopolitically significant location, and its role as the world's focal point of Shia Islam. Iran is a threshold state with one of the most scrutinized nuclear programs, which it claims is solely for civilian purposes; this claim has been disputed by Israel and the Western world. Iran is a founding member of the United Nations, OIC, OPEC, and ECO as well as a current member of the NAM, SCO, and BRICS. Iran has 29 UNESCO World Heritage Sites (the 10th-highest in the world) and ranks 4th in intangible cultural heritage or human treasures.

The earliest known presence of hominins in Iran dates to around 800,000 BP, in the Middle Paleolithic. Many Middle Paleolithic sites have been discovered, mainly in the Zagros Mountains in western Iran and some sites associated with Neanderthals. The Zarzian culture is documented in Iran during the Epipaleolithic (25,000–11,500 BP). Agriculture first appeared in Iran some 12,000 years ago alongside better-documented settlements in the Fertile Crescent. The Chogha Golan site featured early domestication of emmer wheat, The contemporary site of Ganj Dareh also features the earliest known domestication of goats around 10,000 years ago.

The ancient city of Susa, which would become the capital of Elam and later a capital city of the Achaemenid empire, was first settled in 4400–4200 BC, adjacent to the modern site of Shush, Iran. The Kura–Araxes culture (c. 3400 – c. 2000 BC) existed in northwestern Iran and the Caucasus.

Iran is home to one of the world's oldest continuous major civilizations, with historical and urban settlements dating back to 4000 BC, including the Jiroft culture in southeastern Iran. Inscriptions in the Proto-Elamite script, which predates cuneiform, have been found from during the early third millennium BC. The western part of the Iranian plateau participated in the traditional ancient Near East with Elam (3200–539 BC), and later with other peoples such as the Kassites, Mannaeans, and Gutians. The earliest Iranian peoples began to arrive from Central Asia in the 2nd millennium BCE.

The Median dynasty ruled the earliest Iranian state. In 612 BC, Cyaxares and the Babylonian king Nabopolassar invaded Assyria and destroyed Nineveh, the Assyrian capital, which led to the fall of the Neo-Assyrian Empire. The Medes later conquered and dissolved Urartu as well.