The study examined the Optimization of Pavement Design Using Recycled Plastic Waste for Sustainable Road Construction in Uganda with the aim of enhancing pavement performance while addressing the environmental challenge of plastic waste accumulation. The research specifically sought to characterize and optimize the material properties of plastic-modified bitumen using locally sourced recycled plastic waste and Ugandan bitumen; to evaluate the mechanical performance and durability of the optimized plastic-modified asphalt concrete mix under simulated Ugandan environmental and traffic conditions; and to assess the technical, economic, and environmental feasibility of implementing the optimized pavement design for large-scale adoption in Uganda. Laboratory experiments were conducted using penetration, softening point, ductility, specific gravity, and flash point tests on both conventional and plastic-modified bitumen at varying plastic contents of 5%, 10%, and 15%. Marshall stability tests were also carried out to assess the strength and flow characteristics of asphalt concrete mixtures. The results indicated that the incorporation of recycled plastic waste into bitumen significantly improved its physical and thermal properties. The penetration value decreased from 63 mm in conventional bitumen to 49 mm at 10% plastic content, while the softening point increased from 46.2°C to 57.6°C, showing enhanced stiffness and temperature resistance. Ductility slightly reduced but remained within acceptable limits, confirming that flexibility was not compromised. The optimum performance was observed at 10% plastic content, which provided the best balance between stiffness and flexibility. The Marshall stability of the asphalt concrete increased from 12.4 kN for conventional asphalt to 16.7 kN for the 10% plastic mix, and the Marshall quotient improved from 3.26 to 5.21 kN/mm, indicating higher load-bearing capacity and resistance to deformation. Additionally, durability tests revealed improved resistance to moisture damage and stripping, making the mix suitable for Uganda’s tropical climate. Economically, the study found that while the initial production cost of plastic-modified asphalt was about 12.5% higher than conventional asphalt, the long-term benefits were significant. Maintenance costs were reduced by approximately 42%, and the service life increased from 8 to 13 years, yielding substantial cost savings over the pavement’s lifespan. Environmentally, each kilometer of plastic-modified road consumed about 1,200 kilograms of recycled plastic waste and reduced carbon emissions by 31%, demonstrating the sustainability of the technology. It was concluded that the use of recycled plastic waste in bitumen modification improved pavement performance, extended service life, and contributed to environmental conservation. The optimized mix at 10% plastic content was found to be technically feasible, economically viable, and environmentally sustainable for large-scale implementation in Uganda. It was recommended that the Ministry of Works and Transport and the Uganda National Roads Authority develop national standards and guidelines for the use of plastic-modified asphalt. Pilot projects should be conducted in different climatic zones to evaluate long-term field performance, and capacity-building programs should be introduced for engineers and contractors. Furthermore, plastic waste collection systems should be strengthened, and incentives should be provided to encourage private sector investment in recycling for road construction.