Philolaus was a Greek philosopher from Croton in southern Italy, active in the late 5th century BCE. He belonged to the Pythagorean school of thought. Historical details of his life are sparse, but ancient sources suggest he fled political unrest in Croton and later lived in Thebes. He is significant for being the first known Pythagorean to write down the group's doctrines, which were previously shared only orally.

He is traditionally credited with authoring a book, often called On Nature. The surviving fragments of his work, preserved by later philosophers, are believed to possibly come from a single text, though its exact title and structure are uncertain.

Philolaus holds a pivotal place in the history of philosophy. By committing Pythagorean ideas to writing, he helped transmit them to major later thinkers like Plato and Aristotle. His philosophical system was built on the interaction of two fundamental principles: the Unlimited and the Limiting, which combine to create a harmonious world-order. He also proposed an influential cosmic model where the Earth, Sun, Moon, and planets revolved around a central fire—an important step away from Earth-centered cosmology. According to modern scholars, his ideas significantly influenced Plato's work and, through it, much of the Western philosophical tradition.