The increasing accumulation of plastic waste and the persistent durability challenges associated with conventional asphalt pavements have prompted the search for sustainable material modifications. Among potential additives, recycled polyethylene terephthalate (RPET) has emerged as a promising modifier capable of enhancing pavement performance while supporting environmental sustainability. This study investigated the mechanical and microstructural behavior of hot-mix asphalt (HMA) modified with RPET using a drying process. RPET was incorporated at proportions ranging from 0% to 10% of the total mix mass, and the mixtures were evaluated through Marshall stability and flow, uniaxial compressive strength, indirect tensile strength, rutting resistance, dynamic modulus, semi-circular bending, moisture sensitivity, and scanning electron microscopy. Results indicate that RPET significantly improves HMA performance up to an optimal content of 8%. At this dosage, Marshall stability increased from 6.40 to 11.97 kN, while flow decreased from 11.67 to 5.17 mm, demonstrating enhanced stiffness and resistance to permanent deformation. UCS and ITS rose from 1.10 to 1.85 MPa and 0.165 to 0.278 MPa, respectively, and rutting depth declined from 5.0 to 3.0 mm. Additionally, the dynamic modulus increased from 1500 to 2500 MPa, and the SCB increased from 320 to 590 J/m², confirming the enhanced cracking resistance. SEM analysis revealed stronger binder–aggregate interaction at intermediate RPET levels, whereas excessive RPET (10%) caused particle agglomeration and slight performance reductions. The findings show that RPET improves hot mix asphalt mainly through physical reinforcement and microstructural densification, with optimal dosage offering a sustainable way to enhance pavement durability while reducing plastic waste.