Athamas of Posidonia was a philosopher from the 5th century BCE. He lived in the Greek colony of Posidonia, known in Latin as Paestum, in southern Italy. This region was a major center for the Pythagorean school of thought, and his location strongly supports his classification as a Pythagorean philosopher.

No specific details of his life or teachings have survived in the historical record. While he is credited with a treatise, no fragments or titles of his work are known. Major scholarly references and digital libraries contain no direct entries or mentions of him or his writings.

According to modern scholars, his significance lies primarily in his context. As a Pythagorean from Magna Graecia, he would have been part of a movement influential in early mathematics, cosmology, and religious thought. However, his complete absence from surviving sources suggests he was a very minor or local figure within that tradition, and his specific contributions are lost.