The construction industry is responsible for substantial resource consumption and waste generation, particularly from demolished concrete structures. Recycled concrete aggregates (RCA) offer a sustainable alternative to natural aggregates in reinforced concrete (RC) production. However, concerns regarding reduced mechanical strength, increased porosity, and durability performance limit widespread structural adoption. This study investigates the structural behaviour of reinforced concrete beams incorporating varying proportions of recycled aggregates (0%, 25%, 50%, 75%, and 100%) as partial replacement of coarse natural aggregates. Experimental evaluation focuses on load–deflection response, ultimate load capacity, cracking behaviour, stiffness degradation, and failure modes under four-point bending. Results indicate that moderate RCA replacement (≤50%) produces marginal reductions in ultimate strength while maintaining ductile failure characteristics. Higher replacement ratios reduce stiffness and increase deflection due to weaker interfacial transition zones (ITZ). The study concludes that structural-grade RC beams with up to 50% recycled aggregate substitution can satisfy serviceability and strength requirements when mix design adjustments are properly implemented. These findings contribute to sustainable structural engineering practices and support circular economy integration within reinforced concrete construction.