Dionysius Thrax (Ancient Greek: Διονύσιος ὁ Θρᾷξ) (170 BC – 90 BC) was a Hellenistic grammarian and a pupil of Aristarchus of Samothrace. His place of origin was not Thrace as the epithet Thrax denotes, but probably Alexandria. He lived and worked in this city but later taught at Rhodes (around 144BC).
The first extant grammar of Greek, "Art of Grammar" (Tékhnē grammatiké, Greek: τέχνη γραμματική) is attributed to him but many scholars today doubt that the work really belongs solely to him due to the difference between the technical approach of most of the work and the more literary approach (similar to the 2nd century's Alexandrian tradition) of the first few sections. It concerns itself primarily with a morphological description of Greek, lacking any treatment of syntax. The work was translated into Armenian and Syriac in the early Christian era.
Thrax defines grammar at the beginning of the Tékhnē as "the practical knowledge of the general usages of poets and prose writers." Thus Thrax, like contemporary Alexandrian scholars who edited Attic Greek and Homeric texts, was concerned with facilitating the teaching of classic Greek literature to an audience who spoke Koine Greek.