The Collaborative International Dictionary of English v.0.48:

Congregationalist \Con`gre*ga"tion*al*ist\, n.
   One who belongs to a Congregational church or society; one
   who holds to Congregationalism.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Congregationalist
    adj 1: of or pertaining to or characteristic of a Congregational
           church [syn: Congregational, Congregationalist]
    n 1: a member of the Congregational Church