Stiffness of pavement subgrade materials that is commonly determined by the resilient modulus parameter is an important component in the mechanistic design of flexible pavement structures. Environmental effects such as seasonal variations in pavement moisture content can considerably influence this material property which should be properly considered in any realistic pavement design. The seasonal changes in moisture condition affects the stress state in the subgrade due to changes in the matric suction which is an important stress state variable in unsaturated soil mechanics. In this study, a modified test procedure and a predictive resilient modulus model that takes into account the subgrade soil matric suction as a stress state variable is presented. Two different silty sand subgrade materials were tested in unsaturated conditions using a series of Repeated Load Triaxial (RLT) tests. The tests were performed under various matric suctions (moisture contents) to enhance the understanding of its effect on the resilient modulus. The results showed a considerable influence of the moisture content (matric suction) on the subgrade resilient modulus. The resilient modulus data together with the suction measurements were used and a set of parameters for the enhanced predictive model were developed. This model accounts for seasonal variation of subgrade material stiffness by incorporating suction as a stress state variable.