Grammaticus is the Latin word for grammarian; see Grammarian (Greco-Roman world).
As an agnomen, it may refer to:
Ammonius Grammaticus (4th century), Greek grammarianDiomedes Grammaticus (4th century), Latin grammarianMusaeus Grammaticus (6th century), Greek poetVirgilius Maro Grammaticus (7th century), early medieval Latin writerJohn VII Grammaticus (9th century), Ecumenical Patriarch of ConstantinopleAlfricus Grammaticus (10th century), Anglo-Saxon abbot and authorSaxo Grammaticus (circa 1150-1220), Danish medieval historian
As a pseudonym:
Edward Blaiklock, (1903-1983), British born New Zealand author, wrote as "Grammaticus"