The modelling of viscosity of asphaltene solutions is a longstanding unresolved problem. A number of empirical and semi-empirical viscosity models have been proposed. However, no single equation is found to adequately describe all the asphaltene systems. In this article, a new viscosity model for asphaltene solutions is developed taking into consideration the clustering of asphaltene nano-aggregates. At low concentrations of asphaltenes, the asphaltenes are assumed to exist in the form of isolated disk-shaped nano-aggregates. At high concentrations, clustering of nano-aggregates is allowed, resulting in an increase in the effective volume fraction of asphaltenes due to continuous-phase immobilization within the clusters. A model is proposed to relate the effective volume fraction of asphaltenes to the actual volume fraction. The ratio of effective volume fraction to actual volume fraction of asphaltenes is dependent on the type of packing of nano-aggregates within the clusters. The viscosity model is developed using the effective medium approach, taking into account the clustering of nano-aggregates and the relationship between effective volume fraction and actual volume fraction of asphaltenes. Twenty-one sets of literature data covering different sources of asphaltenes, different types of asphaltenes, different types of solvents, and broad ranges of temperature and volume fraction of asphaltenes, are used to validate the proposed model. All the viscosity data could be described very well with the proposed model assuming random close packing of nano-aggregates within the clusters. Only intrinsic viscosity is required to predict relative viscosity of concentrated asphaltene solutions from the model.