WordNet (r) 3.0 (2006):

theatergoer
    n 1: someone who attends the theater [syn: playgoer,
         theatergoer, theatregoer]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "theatergoer":
   aficionado, attender, audience, buff, claqueur, deadhead, fan,
   frequenter, groundling, habitue, haunter, hired applauder,
   moviegoer, pass holder, patron, playgoer, spectator, standee,
   visitor