Theon of Smyrna was a Greek philosopher and mathematician who lived during the Roman imperial period, likely in the 1st or 2nd century CE. He is called "of Smyrna" to distinguish him from other historical figures with the same name. He was a Platonist philosopher with strong Neopythagorean interests, and his work suggests he served as a teacher or expositor.

His only known surviving work is the Expositio rerum mathematicarum ad legendum Platonem utilium ("Exposition of Mathematical Matters Useful for Reading Plato"). This handbook was designed as a student guide to the four mathematical sciences—arithmetic, music, geometry, and astronomy—which were considered essential preparation for studying Plato's philosophy. The work survives but is incomplete in its original form.

Theon of Smyrna is not considered a major original thinker. According to modern scholars, his significance lies in his role as a compiler. His Expositio is a valuable historical source because it preserves summaries of earlier Greek mathematical and harmonic theories, including Pythagorean doctrines. It provides important evidence for the educational curriculum of Platonist schools in the early Roman Empire and shows how mathematical studies were integrated into philosophical training.