The Collaborative International Dictionary of English v.0.48:

Diarist \Di"a*rist\, n.
   One who keeps a diary. Diarrhea

WordNet (r) 3.0 (2006):

diarist
    n 1: someone who keeps a diary or journal [syn: diarist,
         diary keeper, journalist]

Moby Thesaurus II by Grady Ward, 1.0:

72 Moby Thesaurus words for "diarist":
   Boswell, advertising writer, annalist, art critic, author,
   authoress, autobiographer, autobiographist, belletrist,
   bibliographer, biographer, calendar maker, calendarist, chronicler,
   chronographer, chronologer, chronologist, clockmaker, coauthor,
   collaborator, columnist, compiler, composer, copywriter,
   creative writer, critic, dance critic, drama critic, dramatist,
   encyclopedist, essayist, free lance, free-lance writer, ghost,
   ghostwriter, historian, historiographer, horologist, humorist,
   inditer, literary artist, literary craftsman, literary critic,
   literary man, litterateur, logographer, magazine writer,
   man of letters, memorialist, monographer, music critic,
   newspaperman, novelettist, novelist, pamphleteer, penwoman, poet,
   prose writer, reviewer, scenario writer, scenarist, scribe,
   scriptwriter, short-story writer, storyteller, technical writer,
   timekeeper, timer, watchmaker, word painter, wordsmith, writer