The Collaborative International Dictionary of English v.0.48:

Masquerader \Mas`quer*ad"er\, n.
   One who masquerades; a person wearing a mask; one disguised.
   [1913 Webster]

WordNet (r) 3.0 (2006):

masquerader
    n 1: a participant in a masquerade [syn: masquerader,
         masker, masquer]