Mazdak (in Persian مزدک) (died c. 524 or 528) was a proto-socialistPersian reformer and religious activist who gained influence under the reign of the Sassanian Shahanshah Kavadh I. He claimed to be a prophet of God, and instituted communal possessions and social welfare programs.

Mazdak was the chief representative of a religious and philosophical teaching called Mazdakism, which he viewed as a reformed and purified version of Zoroastrianism, although his teaching has been argued to display influences from Manichaeism as well. Zoroastrianism was the dominant religion of Sassanid Persia, and Mazdak himself was a Zoroastrian priest, or mobed, but most of the Zoroastrian clergy regarded his teaching as a heresy. Information about it is scarce and little is known beyond what is listed below; some further details may be inferred from the later doctrine of Khurramism, which has been seen as a continuation of Mazdakism.

Some sources claim that the original founders of this sect lived earlier than Mazdak. These were another mobed named Zardusht (of the same name, but not the same person as the founder of Zoroastrianism) and/or a Zoroastrian philosopher known as Mazdak the Elder, who taught a combination of altruism and hedonism: "he directed his followers to enjoy the pleasures of life and satisfy their appetite in the highest degree with regard to eating and drinking in the spirit of equality,to aim at good deeds; to abstain from shedding blood and inflicting harm on others; and to practice hospitality without reservation". This doctrine was further developed by the best known Mazdak, Mazdak the Younger, son of Bāmdād. At later stages the opposition to Mazdak blamed them for heresy and sharing women etc. which is not true. They are considered to be the first real socialists of human history who emphasized the community work and benefits for all.