Adamantius was a Jewish physician and medical writer who lived in the late Roman Empire, most likely during the 4th or 5th century CE. His epithet, "the Jew," clearly identifies his religious background, placing him within the educated, Greek-speaking Jewish diaspora of the Eastern Roman Empire. Beyond this, no specific details of his life or education are known.

He is credited with authoring medical treatises, the most significant of which is the Physiognomonica. This work deals with physiognomy, the practice of assessing a person's character from their physical features. The treatise is largely a compilation and adaptation of earlier sources, particularly the work of the 2nd-century writer Polemon of Laodicea. The attribution of other medical works to Adamantius is less certain, but the Physiognomonica survives and has been published in modern editions.

Adamantius is a notable figure as one of the few known Jewish authors in the Greek medical tradition of the Roman period. According to scholars, his work demonstrates how Jewish intellectuals participated in and contributed to the broader Hellenistic scientific culture. His main significance lies in his role as a compiler, helping to preserve and transmit earlier physiognomic theories during a period known for systematizing classical knowledge. His existence highlights the multicultural nature of late antique scholarship.