Adamantius the Sophist was a Christian author who lived during the late Roman Empire, likely in the 4th or 5th century CE. His true identity is uncertain; "Adamantius" was a common pseudonym meaning "unconquerable," also used by the famous theologian Origen. The title "sophist" suggests he was a teacher of rhetoric, though this may be a literary convention rather than a statement of his profession.

He is known for a single surviving work, a theological dialogue titled De recta in Deum fide ("On the True Faith in God"). This text is written from an orthodox Christian perspective and is structured as a debate between the orthodox Adamantius and proponents of various early Christian movements considered heretical, such as the Marcionites and followers of Bardaisan. The dialogue is set in the late 4th century, reflecting the intense theological controversies of that period.

Modern scholars regard this work as an important source for understanding early Christian debates and heresiology. It provides detailed, though polemical, accounts of rival doctrines and illustrates the arguments used by defenders of the Nicene Creed. The dialogue also preserves fragments from other otherwise lost texts, adding to its historical value. Despite the mystery surrounding its author, the work offers a significant window into the religious conflicts of late antiquity.