Ammonius the Grammarian was a scholar of the Greek language who lived during the 4th century CE, a period known as Late Antiquity. He was active in Alexandria, Egypt, a major center of learning where pagan scholars worked to preserve classical knowledge. His specific role was that of a grammarian, meaning he specialized in the rules and nuances of the Greek language.

He is best known for a surviving work titled On Similar and Different Words, which is a lexicon examining synonyms and precise word usage. A second, less clearly understood work attributed to him is On Improper Nouns or Expressions, though its exact content and whether it survives are uncertain.

Ammonius represents an important scholarly tradition of his time called Atticism, which sought to define and uphold the classical Athenian Greek of authors like Plato and Demosthenes as the standard for educated writing. According to modern scholars, his surviving lexicon is a valuable resource for understanding ancient Greek vocabulary, etymology, and the methods of late antique philology. His work also indirectly preserves citations and usages from older texts that are now lost, contributing to our knowledge of the Greek literary tradition.