Keyumars or Kiomars (کیومرث) was the name of the first king (shah) of the Pishdadian dynasty of Iran according to the Shahnameh.
The name appears in Avestan in the form of 𐬔𐬀𐬌𐬌𐬊 𐬨𐬆𐬭𐬆𐬙𐬀𐬥 Gaiio Mərətan, or in medieval Zoroastrian texts as Gayōmard or Gayōmart. In the Avesta he is the mythological first human being in the world. The corresponding name in Middle Persian is 𐭪𐭣𐭬𐭫𐭲 Kayōmart. In Ferdowsi's Shahnameh he appears as the first shah of the world. He is also called the pišdād (پيشداد), the first to practice justice, the lawgiver.
The Avestan form means "the living mortal", from gaya "life" and marətan "mortal, human being"; cf. Persian mard "human" (مَرد).
Keyumars is also a popular first name in Persian speaking countries (Iran, Afghanistan and Tajikistan).
According to the Zoroastrian creation myth, Gayōmart was the first human, or, according to the Avesta, he was the first person to worship Ahura Mazda. The Avestan forms Mashya and Mashyana appear as the male and female first humans; their names are versions of the word marətan "mortal".
In the eighth book of the Denkard, a reference is made to the Chethrdāt section of the Avesta, which is divided into 21 sections. Apparently this section dealt with how the world and mankind were created, including the creation of Gayōmart. References are also made to the Varshtmānsar section, which also included information about Gayōmart which Ahura Mazda had given to Zoroaster: "For 30 centuries I kept the world from corruption and decay, when the 30th century came to an end the Dīvs assaulted Gayōmart ... But I finally repelled them and plunged them into the darkness".
A concise story of Gayōmart according to Middle Persian texts is given by Zabihollah Safa:
Gayōmart Gar-shāh (King of the Mountains) was the first human Uhrmazd created. Before Gayōmart came, in the fifth "Gāh" (Ahura Mazda created the world in six Gāhs) Gavevagdāt (the primordial ox) had been created from mud in Erān-vēdj (which was the middle of the earth) on the right side of the river "Veh-Dāit" ...