Ostanes the Alchemist is a semi-legendary figure from the late antique period, traditionally described as a Persian magus and sage. His historical existence is difficult to verify. Ancient sources often place him in the court of the Persian king Xerxes I in the 5th century BCE, but scholars generally agree that the figure is a later construct. The writings attributed to him were likely composed much later, between the 1st and 4th centuries CE.

He is presented as a foundational authority in the Greco-Egyptian alchemical tradition. His constructed persona bridges Persian and Egyptian wisdom, embodying the syncretic nature of late antique esoteric sciences. Later alchemical authors, such as Zosimus of Panopolis around 300 CE, frequently cited his teachings to lend their own work authority and an aura of ancient, revealed knowledge.

No complete works by Ostanes survive. His ideas are known only through fragments, quotations, and references in later texts. These fragments discuss alchemical concepts like the four elements, the transmutation of metals, and the mystical properties of stones and plants. According to modern scholars, his historical importance lies not in a real individual, but in his role as a powerful pseudepigraphic archetype. He became a central name in the narrative of how alchemical knowledge was transferred from the East to the Greek world, influencing the development of alchemical theory and Hermetic thought.