The Collaborative International Dictionary of English v.0.48:

Rationalistic \Ra`tion*al*is"tic\
   (r[a^]sh"[u^]n*al*[i^]s"t[i^]k), Rationalistical
\Ra`tion*al*is"tic*al\ (-t[i^]*kal), a.
   Belonging to, or in accordance with, the principles of
   rationalism. -- Ra`tion*al*is"tic*al*ly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rationalistic
    adj 1: of or relating to the philosophical doctrine of
           rationalism

Moby Thesaurus II by Grady Ward, 1.0:

68 Moby Thesaurus words for "rationalistic":
   Cyrenaic, Eleatic, Epicurean, Megarian, Stoic, allegorizing,
   animist, animistic, annotative, atomistic, clarifying,
   cosmotheistic, critical, demonstrative, demythologizing, eclectic,
   editorial, elucidative, empirical, enlightening, eudaemonistic,
   euhemeristic, exegetic, exemplificative, existential, explaining,
   explanatory, explicative, explicatory, expositive, expository,
   glossarial, hedonic, hedonist, hedonistic, humanist, humanistic,
   idealistic, illuminating, illuminative, illustrative,
   instrumentalist, materialistic, mechanistic, metaphysical,
   monistic, naturalistic, nominalist, panlogistical, pantheistic,
   positivist, positivistic, pragmatic, pragmatist, rationalizing,
   realist, realistic, scholastic, scholiastic, sensationalistic,
   syncretistic, theistic, transcendentalist, transcendentalistic,
   utilitarian, vitalistic, voluntarist, voluntaristic