Philolaus (flourished c. 475 bc) was a philosopher of the Pythagorean school, named after the Greek thinker Pythagoras (fl. c. 530 bc).
Philolaus was born either at Tarentum or, according to the 3rd-century-ad Greek historian Diogenes Laërtius, at Croton, in southern Italy. When, after the death of Pythagoras, dissension was prevalent in Italian cities, Philolaus, according to some accounts, fled first to Lucania and then to Thebes, in Greece. He later returned to Italy, where he may have been a teacher of the Greek thinker Archytas.
Philolaus was a student of the celebrated number theory of Pythagoras, who stressed the importance of numerical groupings. He was particularly interested in the properties inherent in the decad, the sum of the first four numbers. Speusippus, the successor of Plato as head of the Greek Academy, is reported to have reproduced the doctrine of the first four numbers from a book by Philolaus. Only fragments of his works survive, however, and the belief that Philolaus was the first systematizer of Pythagoreanism is widely disputed.