<p>The synergistic use of steel slag and marble dust provides an environmentally friendly alternative to conventional aggregates in bituminous mixtures. By incorporating these industrial by-products, this study aims to reduce waste while enhancing the mechanical and durability properties of asphalt mixes. To evaluate their effectiveness, natural aggregate was partially replaced with steel slag in varying proportions (20–100%), and marble dust was used as a filler. The resulting mixtures were assessed through standard tests, including Marshall Stability, Retained Stability, Indirect Tensile Strength, and Tensile Strength Ratio tests. The results identified an optimal mix design in which a 60% substitution of steel slag combined with marble dust filler demonstrated superior strength. This mixture exhibited higher air voids, increased optimum bitumen content, and improved mechanical strength and durability. The observed enhancement is primarily attributed to the synergistic interaction between steel slag’s alkaline nature and marble dust’s acidic character. This interaction stabilizes the pH and promotes the formation of calcium silicate hydrate, leading to stronger interfacial bonding. This composite approach provides a promising pathway toward more durable and sustainable Bituminous Concrete (BC) applications.</p>