Euphantus of Olynthus was a Greek philosopher of the 4th century BCE. He belonged to the Megarian school, a philosophical group known for its emphasis on logic and dialectic. Ancient sources indicate he was a student of Eubulides of Miletus.

His most significant historical role was as the tutor to Antigonus II Gonatas, the future king of Macedon. This position situated him within the intellectual and political circles of the early Hellenistic period. He is credited with authoring several works, none of which survive. These included a philosophical treatise titled On Kingship, a History of His Own Times, and a number of reportedly well-regarded tragedies.

The complete loss of his writings means his specific philosophical doctrines are unknown. Modern scholarship views Euphantus as an important cultural link, illustrating the role of philosophers as educators and advisors to powerful rulers in his era. While his own ideas remain obscure, his affiliation with the Megarian school connects him to a key tradition in the development of logic. His lost treatise On Kingship is considered a potentially valuable missing work in the study of early Hellenistic political thought.