The experimental investigation described in this paper refers to the assessment of the relationship between morphology of crumb rubber particles and viscosity of corresponding asphalt rubber binders. Analyses were carried out on crumb rubber products derived from ambient, cryogenic and high pressure waterjet size-reduction processes. Asphalt rubber binders were prepared with a common base bitumen and crumb rubber dosage. An innovative technique based on the combined use of microscopic observations, image analysis algorithms and three-dimensional particle models was developed for the quantitative evaluation of the morphological characteristics of crumb rubber. Viscosity tests were carried out on binders in a wide temperature range at a given shear rate. Results of morphological analyses were coherent with each other and totally compatible with the information regarding production processes. A clear viscosity ranking of asphalt rubber binders was established, highlighting the peculiarities of different types of crumb rubber. A prediction model was proposed which shows the dependency of viscosity on the morphological characteristics of crumb rubber. The model proved to be statistically sound and in agreement with the interaction phenomena which occur within asphalt rubber binders.