Polyidus of Thessaly (also Polyides, Polydus; Ancient Greek: Πολύειδος ὁ Θεσσαλός, Polúeidos ho Thessalós, English translation: "much beauty", from polus, "many, much" and eidos, "form, appearance, beauty") was an ancient Greek military engineer of Philip, who made improvements in the covered battering-ram (testudo arietaria, poliorceticus krios) during Philip's siege of Byzantium in 340 BC. His students were Diades of Pella and Charias, who served in the campaigns of Alexander the Great.