Kay Bahman or Wahman (from Middle Persian: Wahman "good mind"; Persian: کی‌بهمن) is a mythological figure from the national history of Greater Iran. The stock epithet Kai identifies Bahman as one of the Kayanian kings of Iranian oral tradition.

The 3rd-6th century Sassanians claimed to descend from Bahman and the Kayanids. This myth was combined with another legend in which the Sassanians were imagined to have descended from the Achaemenids, and in the post-Sassanid period Bahman came to equated with both Artaxerxes I and Cyrus the Great.

In the genealogy of the legendary Kayanian dynasty, Bahman is the son of Esfandiyar (hence his also being referred to in Middle Persian as 'Vohuman Asfandyar'), grandson of Goshtasp, husband of [daughter of the Egyptian king ] Komani/Homai, and father of Dara/Darab. Other details vary: Several different Arabic and Persian sources (e.g. al-Tabari and ibn al-Balkhi) assert that Bahman had five children; two sons, Dara/Darab and Sasan/Sassan, and three daughters, Komani/Homai (stock epithet: Chehrazad), Bahmandokht and Farnak.

In the Bahman-nama (composed ca. 1100 CE, not to be confused with a 15th-century hagiography of the Deccan Bahmanids) Bahman is portrayed as being asked by Rostam to marry Katayun/Kasayun, daughter of the King of Kashmir. Bahman does so, but subsequently has to flee to Egypt, where he marries Homai, the daughter of the Egyptian king, with whom he has a daughter also named Homai. Other sources have Bahman marrying his own daughter Komani/Homai on account of her great beauty, and that Dara/Darab was the result of this union. In one tradition (reiterated by ibn al-Balkhi), the marriage was denied and Homai dies a spinster.

The name 'Bahman'/'Wahman' is a theophoric reflecting Middle Persian Vohuman, Avestan Vohu Manah, the Amesha Spenta (Ameshaspand) that is the hypostasis of the "Good Mind" or "Good Disposition." Unlike most figures of Iranian oral tradition, Bahman does not however appear in the surviving Avesta. He is however referred to in the Zand-i Wahman yasn (also known as the Bahman Yasht, which—despite that name—is not an Avestan Yasht, but a medieval pseudo-prophetic/apocaplyptic text). The figure is also mentioned in the 9-11th century texts of Zoroastrian tradition, specifically, the Middle Persian Bundahishn (GBd 36.9) and the Denkard (VII.6), both of which enumerate the Kayanian kings. In both the Zand-i Vohuman Yasht (3.20-29) and the Denkard, Bahman is described as "one of the greatest Mazdayasnian kings of Iran."

"The importance attached to Bahman's reign in Zoroastrian literature can be seen in a passage in the Bahman yašt where the reigns of the kings are likened to tree branches made of seven metals." In this comparison, which lists the reigns in order of perceived importance, the 'silver' reign of Bahman appears second, after the 'golden' reign of (likewise mythological) Goshtasp and before the 'brass' reign of the Sassanid Ardashir I, the 'bronze' reign of the Arsacid Balash, the 'tin' reign of the Sassanid Bahram V Gor, the 'steel' reign of the Sassanid Khosrow I Anushirvan, with 'iron' (which is considered impure in Zoroastrianism) representing the "rule of the shaggy-haired demons (dēws) born of the seed of anger, probably referring to the rule of the Arabs."

In Firdausi's Shahnameh (ca. 1000 CE), which versifies and embellishes previous tradition, Bahman's father Esfandiar is portrayed as having been killed by Rostam. The Shahnameh goes on to describe how Bahman sets off with a large army to avenge his father's death. In the meanwhile however, Rostam had already been treacherously killed by Rostam's half-brother Shaghad. Frustrated, Bahman instead kills Rostam's son Faramarz, imprisons Rostam's father Zal (whom he however releases on the behest of his uncle Peshotan), and lays waste to Rostam's feudal lands.

Again, the details in other sources vary. According to al-Tabari, al-Masudi and al-Atir, Bahman himself killed not only Rostam but Faramarz, Zal and Rostam's brother Zavara as well. Ibn al-Balkhi includes only Faramarz and Zal. The Bahman-nama has Bahman mourning for Rostam, who—along with Zavara—are described as having been killed by the king of Kabul. In this text, Bahman does however travel to Rostam's (now Faramarz's) fiefdom, where he battles Rostam's son (who then flees) but where he takes Zal prisoner. Bahman then pursues Rostam's daughter's Banu Goshasp and Zar Banu to Kashmir, and then Faramarz to India, where Faramarz is finally slain. Having captured Rostam's/Faramarz's lands, Bahman then seeks to destroy the tombs of Rostam and his ancestors (Garshasp, Nariman and Sam). At each of the respective tombs he receives a precious gift and a message asking him to stay his hand, which he does. Chastened, he then frees Zal and Rostam's daughters, and rebuilds everything he had previously destroyed. In the Bahman-nama, Bahman is killed by a dragon while out hunting. In Iranian tradition, such a fate is typically reserved for villains, and this is the only instance in which a legitimate king is said to have been killed in such a fashion.