Adrastus (Greek: Ἄδραστος; 2nd century) of Aphrodisias was a Peripatetic philosopher who lived in the 2nd century AD. He was the author of a treatise on the arrangement of Aristotle's writings and his system of philosophy, quoted by Simplicius, and by Achilles Tatius. Some commentaries of his on the Timaeus of Plato are also quoted by Porphyry, and a treatise on the Categories of Aristotle by Galen. None of these have come down to us. He was a competent mathematician, whose writings on harmonics are frequently cited by Theon of Smyrna in the surviving sections of his On Mathematics Useful for the Understanding of Plato. In the 17th century, a work by Adrastus on harmonics, Περὶ Ἁρμονικῶν ("On Harmonics"), was said by Gerhard Johann Vossius to have been preserved, in manuscript, in the Vatican Library, although the manuscript appears to be no longer extant, if indeed this was not an error on Vossius' part.