Isfahan or Esfahan (Persian: اصفهان [esfæˈhɒːn] ^ⓘ) is a city in the Central District of Isfahan County, Isfahan province, Iran. It is the capital of the province, the county, and the district. It is located 440 kilometres (270 miles) south of Tehran. The city has a population of approximately 2,237,990, making it the fourth-most populous city in Iran, after Tehran, Mashhad and Karaj, and the second-largest metropolitan area.

Isfahan is located at the intersection of the two principal routes that traverse Iran, north–south and east–west. Isfahan flourished between the 9th and 18th centuries. Under the Safavid Empire, Isfahan became the capital of Iran, for the second time in its history, under Abbas the Great. It is known for its Persian–Muslim architecture, grand boulevards, covered bridges, palaces, tiled mosques, and minarets. Isfahan also has many historical buildings, monuments, paintings, and artifacts. The fame of Isfahan led to the Persian proverb Esfahān nesf-e-jahān ast ('Isfahan is half the world'). Naqsh-e Jahan Square in Isfahan is one of the largest city squares in the world, and a UNESCO World Heritage Site.

Isfahan was the second Iranian city to experience the highest volume and number of Israeli airstrikes during the twelve-day war.

The name Espahan is derived from Middle Persian Spahān, which is attested by various Middle Persian seals and inscriptions, including that of the Zoroastrian magi Kartir.

The region is denoted by the abbreviation GD (Southern Media) on Sasanian coins. In Ptolemy's Geographia, it appears as Aspadana (Ἀσπαδανα) or Aspazana (Ασπαζάνα), which translates to "place of gathering for the army". It is believed that Spahān is derived from spādānām "the armies", the Old Persian plural of spāda, from which is derived spāh (𐭮𐭯𐭠𐭧) 'army' and spahi (سپاهی, 'soldier', literally 'of the army') in Central Persian. Some of the other ancient names include Gey, Jey (old form Zi), Park, and Judea.

Human habitation in the Isfahan region can be traced back to the Palaeolithic period. Archaeologists have found artifacts dating back to the Palaeolithic, Mesolithic, Neolithic, Bronze, and Iron ages. During the Median rule, Isfahan became a regional centre, especially from the benefits of the Zayandehrud River. It was a religiously and ethnically diverse city during the reign of Cyrus the Great, and exhibited religious tolerance.

The Arabs captured Isfahan in 642 CE. They made it the capital of al-Jibal province. The city further grew under the Buyid dynasty, and under the Seljuk dynasty. With the fall of the Seljuks in 1200 CE, the city temporarily declined but regained its importance during the Safavid era (1501–1736) with the city's golden age under the rule of Abbas the Great who also moved his capital from Qazvin to Isfahan. During his reign, Turkish, Armenian, and Persian craftsmen were forcefully resettled in the city to ensure its prosperity. Later, the city also had enclaves for people of Georgian, Circassian, and Daghistani descent. The city once again declined after the Siege of Isfahan by Afghan invaders in 1722.

In the 20th century, Isfahan was resettled by many people from southern Iran, especially during the population migrations at the start of the century and in the 1980s following the Iran–Iraq War.