AbstractExperiences of highway agencies show that the costs of repairing rutting distresses are high. The common solution to overcome this issue is using a modified asphalt binder. The modifier enhances the asphalt binder’s rheological and physical properties but increases the production temperatures of asphalt pavement. Nevertheless, warm mix asphalt (WMA) production at temperatures lower than hot mix asphalt (HMA) could lead to lower aging of asphalt binders. In this study, the physical and rheological properties of modified asphalt binders with different contents of Gilsonite and their combination with Sasobit as a WMA agent were investigated. Accordingly, 5%, 9%, and 13% Gilsonite, as well as 3% Sasobit in combination with Gilsonite, were used to modify asphalt binders. Dynamic shear rheometer (DSR), rotational viscosity (RV), surface free energy (SFE), Fourier-transform infrared spectroscopy (FTIR), and some conventional tests (i.e., penetration grade, softening point, and storage stability) were conducted to evaluate the properties of the binders. The results of chemical analysis tests (i.e., SFE and FTIR) indicated that using Gilsonite and Sasobit as composite modifiers could further improve the thermomechanical properties. The DSR test results also showed that the Gilsonite and its combination with Sasobit improve the high-temperature performance of asphalt binders and, consequently, increase the rutting resistance. However, Gilsonite could raise the asphalt binder elasticity and showed better results when Sasobit was incorporated. In general, Gilsonite increased the asphalt binder stiffness, and consequently, it could reduce the workability and compactability of mixtures. Therefore, the use of Sasobit as a complement for Gilsonite reduces the viscosity of the asphalt binder and enhances the mixture workability. This could pave the way to use Gilsonite in WMA, resulting in more durable and sustainable asphalt mixtures.