Bitumen aging leads to increased stiffness, reduced flexibility, and deterioration of asphalt pavement performance throughout its service life. Therefore, the use of rejuvenators and modifying additives to restore the properties of aged binders has attracted considerable attention from researchers. In this study, the combined effects of waste engine oil (WEO) and polyphosphoric acid (PPA) on the rheological and performance properties of SBS-modified aged bitumen were investigated. For this purpose, SBS-modified bitumen was subjected to short-term and long-term aging using the Rolling Thin Film Oven (RTFO) and Pressure Aging Vessel (PAV) procedures. Subsequently, different contents of WEO (0, 8, and 16 wt%) and PPA (0.5, 1.0, and 1.5 wt%) were incorporated into the aged binder. To evaluate the performance of the modified binders, Rotational Viscosity (RV), Dynamic Shear Rheometer (DSR), Bending Beam Rheometer (BBR), Temperature Sweep, and Performance Grade (PG) tests were conducted. The results indicated that the addition of WEO reduced viscosity, improved workability, and enhanced the low-temperature performance of the aged binder. In contrast, PPA increased the complex shear modulus and the rutting parameter (G*/sinδ), thereby improving the binder's resistance to rutting and permanent deformation. Furthermore, temperature sweep results demonstrated that PPA increased the elastic response of the binder, whereas WEO enhanced its flexibility. The evaluation of critical temperatures and PG classification revealed that PPA significantly improved high-temperature performance, while WEO effectively enhanced low-temperature performance. Based on the technical and economic assessment, the combination of 1 wt% PPA and 8 wt% WEO was identified as the optimum formulation. The findings suggest that the simultaneous use of waste engine oil and polyphosphoric acid can serve as an effective approach for restoring and improving the performance of SBS-modified aged bitumen, while also contributing to the development of sustainable technologies in the pavement industry.