Hippasus of Metapontum, born circa 500 B.C. in Magna Graecia, was a Greek philosopher. He is the disciple of Pythagoras to whom is attributed the discovery that the square root of 2 is irrational. After the number was revealed to be irrational, the Pythagoreans killed Hippasus, not wishing to believe this fundamental number could be infinitely long and nonrepeating.

He was also noted as an early experimenter in acoustics and resonance. Few of his original works now survive.