Istakhr (Middle Persian romanized: Stakhr, Persian: استخر, romanized: Estakhr) was an ancient city in Fars province, 5 kilometres (3 mi) north of Persepolis in southwestern Iran. It flourished as the capital of the Persian Frataraka governors and Kings of Persis from the third century BC to the early 3rd century AD. It reached its apex under the Sasanian Empire (224-651 AD), and was the hometown of the Sasanian dynasty. Istakhr briefly served as the first capital of the Sasanian Empire from 224 to 226 AD and then as principal city, region, and religious centre of the Sasanian province of Pars.

During the Arab conquest of Iran, Istakhr was noted for its stiff resistance, which resulted in the death of many of its inhabitants. Istakhr remained a stronghold of Zoroastrianism long after the conquests, and remained relatively important in the early Islamic era. It went into gradual decline after the founding of nearby Shiraz, before being destroyed and abandoned under the Buyids. Cursorily explored by Ernst Herzfeld and a team from the University of Chicago in the first half of the 20th century, much of Sasanian Istakhr remains unexcavated.

"Istakhr" (also spelled Estakhr) is the New Persian form of the Middle Persian Stakhr (also spelled Staxr), and is believed to mean "strong(hold)". According to the Iranologist Ernst Herzfeld, who based his arguments on coins of the Persian Frataraka governors and Kings of Persis, the Middle Persian word in turn derives from Old Persian *Parsa-staxra ("stronghold of Pars"), owing to the city's close connections with the nearby Persepolis platform. Herzfeld interpreted the Aramaic characters "PR BR" inscribed on these coins as an abbreviation of Aramaic prsʾ byrtʾ ("the Fortress of Parsa"), which in turn may be the equivalent of the aforementioned Old Persian words. The abbreviation "ST", denoting Istakhr, also appears on Sasanian coins. Istakhr is attested in Syriac as Istahr and in Armenian as Stahr. It probably appears in the Talmud as Istahar.

Istakhr is located in Iran's southwestern province of Fars, historically known as Parsa (Old Persian), Pars (Middle Persian) and Persis (Greek), whence Persia. It lies in the valley of the Polvar River, between the Kuh-e Rahmat and the Naqsh-e Rostam, where the Polvar River valley opens into the plain of Marvdasht. This plain stretches near the platform of Persepolis.

In all likelihood, what became Istakhr was originally part of the settlements which surrounded the Achaemenid royal residences. Its religious importance as a Zoroastrian center was signified as early as the 4th century BC during the reign of Achaemenid King Artaxerxes II (r. 404-358). During his reign, he ordered the construction of a statue of Anahid and a temple near what would become Istakhr. This temple may be identified with the ruins of the temple mentioned by the 10th-century geographer al-Masudi as being located c. one parasang from Istakhr. According to the Iranologist Mary Boyce, the ruins of this temple probably belonged to the original Achaemenid building, which had been destroyed and pillaged by the invading Macedonians led by Alexander the Great (r. 336–323). Istakhr's foundation as a separate city took place very shortly after the decline of nearby Persepolis by Alexander. It appears that much of Persepolis' rubble was used for the building of Istakhr.

When Seleucus I (r. 305–280) died in 280 BC, the local Persians of Persis began to reassert their independence. The center of resistance appears to have been Istakhr, which with its surrounding hills provided better protection than the nearby former Achaemenid ceremonial capital of Persepolis. Furthermore, an important road, known as the "winter road", extended across Istakhr, leading from Persis to Isfahan through Pasargadae and Abada. The core of Istakhr as a city was located on the south and east side of the Polvar River. It flourished as the capital of the Persian Frataraka governors and Kings of Persis from the 3rd century BC to the early 3rd century AD.

Sasan, the eponymous ancestor of the later Sasanian dynasty, hailed from Istakhr and originally served as the warden of the important Anahid fire-temple within the city. According to tradition, Sasan married a woman of the Bazrangi dynasty, who ruled in Istakhr as Parthian vassals in the early 3rd century. In 205/6, Sasan's son Papak dethroned Gochihr, the ruler of Istakhr. In turn, Papak's sons, Shapur and Ardashir V, ruled as the last two Kings of Persis.

In 224, Ardashir V of Persis founded the Sasanian Empire and became regnally known as Ardashir I (r. 224–242). Boyce states that the temple, which had been destroyed by the Macedonians centuries earlier, was restored under the Sasanians. She adds that according to Al-Masudi, who in turn based his writings on tradition, the temple had "originally been an 'idol-temple', which was subsequently turned into a fire temple by Homay, the legendary predecessor of the Achaemenid dynasty". It appears that in the early Sasanian period, or perhaps a bit before that, the Zoroastrian iconoclastic movement had resulted in the cult-image of Anahid being replaced by a sacred fire. Al-Masudi identified this sacred fire as "one of the most venerated of Zoroastrian fires". The identification of this temple at Istakhr with Anahid persisted, and the historian al-Tabari (died 923) stated that it was known as "the house of Anahid's fire".