Systems, methods, and software for modeling subterranean formation viscosity are described. In some aspects, a method of training a subterranean fluid viscosity model based on NMR data includes accessing multiple relaxation-time distributions generated from NMR measurements of a fluid, normalizing each relaxation-time distribution to a common normalizing value, computing parameters for a plurality of weighted radial basis functions from the normalized relaxation-time distributions, and producing a subterranean fluid viscosity model that includes the weighted radial basis functions and the computed parameters.