The Collaborative International Dictionary of English v.0.48:

Wagerer \Wa"ger*er\ (w[=a]"j[~e]r*[~e]r), n.
   One who wagers, or lays a bet.
   [1913 Webster]

WordNet (r) 3.0 (2006):

wagerer
    n 1: someone who bets [syn: bettor, better, wagerer,
         punter]