Theon of Smyrna was a Platonist philosopher and mathematician active during the reign of Emperor Hadrian. He is distinguished from the later Theon of Alexandria by his association with the city of Smyrna. Beyond this period and his philosophical orientation, no biographical details are recorded.

His only surviving work is the treatise Mathematics Useful for Understanding Plato, a compendium of arithmetic, geometry, harmonics, and astronomy intended as background for reading Plato’s Timaeus. Two other works are attributed to him, On the Order of Plato's Writings and On Mathematical Signs, but these are considered spurious and do not survive.

Theon’s surviving treatise is a valuable source for the state of mathematical astronomy in early Imperial Platonism. It systematically compiles earlier Pythagorean and astronomical knowledge, notably preserving summaries of planetary theories and the work of Adrastus of Aphrodisias. The work illustrates the mathematical curriculum used to approach Platonic cosmology.