(Ἵππασος), of Metapontum or Croton (Iamblich. Vit. Pyth, 18. §§ 81, 88. 23.104), is mentioned both by Iamblichus and by Diogenes Laertius (8.84) among the elder Pythagoreans.

Hippasus is said to have been the founder of a school or sect of the Pythagoreans, called the (ἀκουσματικοί), in opposition to the Aristotle (Aristot. Met. 1.3) speaks of Hippasus as holding the element of fire to be the cause of all things: and Sextus Empiricus (ad Phys. 1.361) contrasts him with the Pythagoreans in this respect, that he believed the ἀρχή to be material, whereas they thought it was incorporeal, namely, number. A single sentence quoted by Diogenes Laertius as expressing one of his doctrines seems to mean that he held all things to be in motion and change, but according to a fixed law. (Iamblich. Ibid. §§ 81, 88; Villoison, Anecd. Graec. ii. p. 216.) In consequence of his making known the sphere, consisting of twelve pentagons, which was regarded by the Pythagoreans as a secret, he is said to have perished in the sea as an impious man.