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