Euphantus of Olynthus was a Greek philosopher who lived during the 4th century BCE. He was a member of the Megarian school, a group of thinkers known for their focus on logic and argument. According to ancient sources, he studied under the philosopher Eubulides of Miletus. His most notable biographical connection was his role as the tutor to Antigonus II Gonatas, who later became king of Macedon. This position placed Euphantus within the intellectual circles of the early Hellenistic period.

Ancient records credit Euphantus with writing several works, though none survive today. These included a philosophical treatise titled On Kingship, a History of His Own Times, and a number of tragedies which were reportedly well-regarded. The loss of these texts means his specific philosophical ideas are not known in detail.

Modern scholars see Euphantus as a significant cultural link. His life illustrates how philosophers in his era could serve as educators and advisors to powerful rulers. While his own doctrines are obscure, his association with the Megarian school connects him to an important tradition in the development of logic. His lost treatise On Kingship is considered a potentially valuable missing perspective on early Hellenistic political thought.