The increasing generation of plastic waste and the growing demand for sustainable pavement materials have encouraged the incorporation of recycled polymers into asphalt mixtures. This study evaluates the engineering performance, microstructural characteristics, numerical response, and preliminary environmental implications of recycled polyethylene terephthalate (RPET)-modified asphalt concrete. RPET obtained from post-consumer plastic bottles was incorporated into asphalt mixtures through the dry process at dosages of 0–9% by weight of binder. Marshall stability, indirect tensile strength (ITS), repeated load dynamic creep (RLDC), scanning electron microscopy (SEM), and finite element modelling (FEM) were employed to assess the influence of RPET content on mixture behavior. Experimental results showed that increasing RPET content improved stiffness-related properties and rutting resistance. Marshall stability increased from 5.5 kN for the control mixture to 14.3 kN at 9% RPET, while ITS increased from 0.72 MPa to 1.02 MPa. RLDC results indicated a reduction in accumulated permanent strain from 3.20% to 1.85%, demonstrating enhanced resistance to deformation under repeated loading. SEM observations revealed comparatively uniform RPET dispersion at moderate dosages (3–5%), whereas higher contents showed localized particle agglomeration. FEM simulations demonstrated reduced surface deflection and improved stress distribution with increasing RPET-related stiffness. Preliminary life cycle assessment indicated modest embodied carbon reduction and potential cost savings. The findings suggest that RPET incorporation can enhance the mechanical and deformation-resistant characteristics of asphalt mixtures while contributing to plastic waste valorization and sustainability objectives. However, the results should be interpreted as comparative laboratory and numerical indicators rather than direct predictors of long-term field performance.