The Collaborative International Dictionary of English v.0.48:

Grammarian \Gram*ma"ri*an\, n. [Cf. F. grammairien.]
   1. One versed in grammar, or the construction of languages; a
      philologist.
      [1913 Webster]

   Note: "The term was used by the classic ancients as a term of
         honorable distinction for all who were considered
         learned in any art or faculty whatever." --Brande & C.
         [1913 Webster]

   2. One who writes on, or teaches, grammar.
      [1913 Webster]

WordNet (r) 3.0 (2006):

grammarian
    n 1: a linguist who specializes in the study of grammar and
         syntax [syn: grammarian, syntactician]

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "grammarian":
   dialectician, etymologer, etymologist, glossarist, glossographer,
   grammaticaster, grammatist, lexicographer, lexicologist, linguist,
   linguistic scholar, linguistic scientist, linguistician,
   orthoepist, paleographer, philologaster, philologer, philologian,
   philologist, phonemicist, phonetician, phoneticist, phonologist,
   semanticist, semasiologist