Low-temperature cracking is a very common type of distress in flexible pavements in cold areas that extends in the form of longitudinal and transverse cracks on the surface layer. Modifying the properties of the control asphalt cement to improve its performance at low temperatures is a way to control this type of cracking. The effect of using ultrahigh-molecular-weight polyethylene (UHMWPE) as a polymeric additive was investigated with rheological, mechanical, and thermodynamic tests. Bending beam rheometer (BBR) testing of the asphalt cement at low temperatures and mechanical tests, including pull off (PO) and semicircular bend (SCB), were performed on asphalt mixtures to examine the effect of this additive on the low-temperature cracking potential of hot-mix asphalt. In addition, thermodynamic tests, including asphalt cement and aggregate surface free energy (SFE) using the Wilhelmy plate (WP) and the universal sorption device (USD) methods, were performed to better understand the results. Based on the results of the BBR test, the application of UHMWPE improved the performance of the modified asphalt cement at low temperatures. Also, the results of the SFE test showed that using UHMWPE increased the cohesion and the adhesion free energies. These changes were closely correlated with the results of the PO test in cohesive and adhesive modes, respectively. The results of SCB test for parameters including peak load, fracture energy, and fracture toughness demonstrated that the incorporation of UHMWPE significantly improves the low-temperature properties of the modified asphalt mixture. Moreover, based on the statistical analysis, the SFE parameters had a positive effect on peak load and fracture energy. The results for the fracture energy parameter revealed that, after crack formation, only the cohesion free energy parameter was effective on asphalt concrete resistance until the moment of failure.