Ecphantus was a Pythagorean philosopher believed to have lived during the 4th century BCE. He is a little-known figure, and no details about his personal life or origins survive. His identification as a follower of the Pythagorean school is consistent in the ancient sources that mention him.

No complete writings by Ecphantus exist. His philosophical ideas are preserved only in fragments and reports within later collections of philosophical opinions, primarily the work known as De Placitis Philosophorum (Opinions of the Philosophers), which was once attributed to Plutarch.

According to these reports, Ecphantus contributed to Pythagorean cosmology. He is said to have taught that the universe is spherical, animated by a divine mind, and governed by providence. Modern scholars note that he proposed the fundamental units composing the cosmos were indivisible, corporeal bodies. This idea has been interpreted as a step toward atomistic theory within the Pythagorean tradition. His historical significance lies in being a named link in the development of Pythagorean thought between its early period and the Hellenistic age, though the fragmentary evidence makes his precise contributions difficult to assess. The attribution of a separate work On Kingship to him is uncertain and not widely confirmed.