The Collaborative International Dictionary of English v.0.48:

Negotiator \Ne*go"ti*a`tor\, n. [L.: cf. F. n['e]gociateur.]
   One who negotiates; a person who treats with others, either
   as principal or agent, in respect to purchase and sale, or
   public or private compacts.
   [1913 Webster]

WordNet (r) 3.0 (2006):

negotiator
    n 1: someone who negotiates (confers with others in order to
         reach a settlement) [syn: negotiator, negotiant,
         treater]

Moby Thesaurus II by Grady Ward, 1.0:

42 Moby Thesaurus words for "negotiator":
   agent, arbitrator, bargainer, broker, business agent, cardholder,
   connection, contact, deputy, front, front man, go-between,
   interagent, interceder, intercessor, intermediary, intermediate,
   intermediate agent, intermediator, intermedium, internuncio,
   interpleader, intervener, interventionist, interventor,
   labor organizer, mediator, medium, middleman, mouthpiece,
   negotiant, negotiatress, negotiatrix, ombudsman, organizer,
   shop steward, spokesman, spokeswoman, trade unionist, union member,
   union officer, unionist