The Collaborative International Dictionary of English v.0.48:

Sociologist \So`ci*ol"o*gist\, n.
   One who treats of, or devotes himself to, the study of
   sociology. --J. S. Mill.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sociologist
    n 1: a social scientist who studies the institutions and
         development of human society