Oenopides of Chios was a Greek mathematician and astronomer active in the 5th century BCE. He is considered part of the Presocratic tradition of thinkers, though his work focused more on geometry and astronomy than on speculative philosophy. Later ancient sources sometimes incorrectly linked him to Pythagoras, which reflects a later tendency to organize early thinkers into schools. His historical details are sparse, and he is known only through reports by later writers.

He did not leave behind a surviving book. Instead, his ideas are preserved as fragments in the works of later commentators. According to these sources, Oenopides made two major astronomical contributions: he is credited with discovering the tilt of the Earth's axis (the obliquity of the ecliptic) and with calculating a 59-year cycle for the sun and moon, known as a "Great Year." In geometry, he was known for solving practical construction problems, such as drawing a perpendicular line from a point to a given line.

Modern scholars see Oenopides as a significant transitional figure. His work represents an early move toward applying mathematical methods to astronomy, paving the way for the more systematic science developed by later Greeks. He exemplifies the shift from cosmological speculation to geometrical investigation in the 5th century BCE.