A computational study into the molecular origin of viscosity in ionic liquids via a combined quantitative structure–property relationship (QSPR) and molecular dynamics (MD) approach is presented. QSPR models are developed for ionic liquids containing the bis[(trifluoromethyl)sulfonyl]imide (TFSI) anion using literature data. The best models thus developed are used to identify persistently well-correlating molecular descriptors for further investigation by MD. By modifying the force field of a simple ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF6], the efficacy of the charged partial surface area (CPSA) descriptor is examined. Finally, by building QSPR models from the MD data and calculations derived from the CPSA descriptor, potential routes for improved prediction of ionic liquid viscosity are proposed.