The Collaborative International Dictionary of English v.0.48:

Alienist \Al"ien*ist\, n. [F. ali['e]niste.]
   One who treats diseases of the mind. --Ed. Rev.
   [1913 Webster]

WordNet (r) 3.0 (2006):

alienist
    n 1: a psychiatrist and specialist in the legal aspects of
         mental illness

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "alienist":
   Adler, Freud, Horney, Pavlov, Reich, Skinner, Watson, alienism,
   clinical psychologist, industrial psychologist, neuropsychiatrist,
   psychiatrist, psychiatry, psychobiologist, psychochemist,
   psychographer, psychologist, psychologue, psychopathist,
   psychopathologist, psychophysicist, psychophysiologist,
   psychotechnologist, somatist