Zoroaster, also known as Zarathustra, is regarded as the spiritual founder of Zoroastrianism. He is said to have been an Iranian prophet who founded a religious movement that challenged the existing traditions of ancient Iranian religion, and inaugurated a movement that eventually became a staple religion in ancient Iran. He was a native speaker of Avestan and lived in the eastern part of the Iranian plateau, but his exact birthplace is uncertain.

There is little scholarly consensus on when he lived. Some scholars, using linguistic and socio-cultural evidence, suggest a dating to somewhere in the second millennium BC. Other scholars date him to the 7th and 6th centuries BC as a near-contemporary of Cyrus the Great and Darius the Great. Zoroastrianism eventually became the official state religion of ancient Iran—particularly during the era of the Achaemenid Empire—and its distant subdivisions from around the 6th century BC until the 7th century AD, when the religion itself began to decline following the Arab-Muslim conquest of Iran. Zoroaster is credited with authorship of the Gathas as well as the Yasna Haptanghaiti, a series of hymns composed in his native Avestan dialect that comprise the core of Zoroastrian thinking. Little is known about Zoroaster; most of his life is known only from these scant texts. By any modern standard of historiography, no evidence can place him into a fixed period and the historicization surrounding him may be a part of a trend from before the 10th century AD that historicizes legends and myths.