Although bitumen constitutes only a small proportion of hot mix asphalt (HMA), it plays a critical role in determining pavement performance and durability. Deficiencies in bitumen properties can lead to various pavement distresses, including rutting, cracking, and fatigue failure. This study investigates the effect of high-density polyethylene (HDPE, Grade 5218) on the physical, rheological, and performance characteristics of bitumen and its influence on the rutting resistance of HMA. HDPE was incorporated into the base bitumen at concentrations of 3%, 5%, 6%, and 7% by weight of binder. The modified binders were evaluated through penetration, softening point, ductility, and rotational viscosity tests, while the performance of the corresponding asphalt mixtures was assessed using Marshall stability and dynamic creep tests. The results demonstrated that increasing HDPE content significantly enhanced the stiffness and deformation resistance of the binder. The penetration value decreased from 64 to 38 dmm, whereas the softening point increased from 51.4°C to 69.5°C for the mixture containing 7% HDPE. In addition, the rotational viscosity increased from 0.374 to 1.517 Pa·s, indicating improved resistance to flow and permanent deformation at elevated temperatures. Marshall test results revealed a substantial increase in stability from 1056 kg to 1828 kg, while flow values remained nearly unchanged. Dynamic creep testing showed that the cumulative permanent strain of the mixture containing 7% HDPE decreased by 46.3% and 46.0% at 40°C and 50°C, respectively, compared with the control mixture, demonstrating a significant improvement in rutting resistance. One-way analysis of variance (ANOVA) confirmed that HDPE content had a statistically significant effect on penetration, softening point, rotational viscosity, Marshall stability, and permanent deformation characteristics (P < 0.001). Based on the experimental findings, the incorporation of 7% HDPE was identified as the optimum dosage for enhancing binder properties and improving the rutting resistance of asphalt mixtures.