Dionysius the Thracian was a Greek grammarian who lived and worked during the 2nd century BCE. Originally from Thrace, he was active in the major intellectual center of Alexandria, where he was a student of the famous scholar Aristarchus of Samothrace. He is considered a key figure in the tradition of Alexandrian scholarship.

His lasting importance rests on a single, short work titled Téchnē Grammatikḗ (The Art of Grammar). This text is regarded as the first systematic grammar of the Greek language and became a foundational model for all subsequent Western grammar. In it, Dionysius defined grammar as the study of literary language and outlined its core components. Most significantly, the work provided the first clear definitions for the eight parts of speech in Greek: noun, verb, participle, article, pronoun, preposition, adverb, and conjunction. According to modern scholars, this framework was enormously influential, shaping the study of Latin and later European languages for centuries.

While the Suda, a later Byzantine encyclopedia, credits him with other works on poetry, these are lost and their exact nature is uncertain. The text of The Art of Grammar that survives has likely been modified by later copyists and scholars, but its essential structure and ideas are attributed to Dionysius. As the oldest surviving grammatical handbook from antiquity, his work established the core terminology and methodology of the field, making him a foundational figure in the history of linguistics and education.