The rising interest in sustainable construction has driven the exploration of alternative aggregates (AA) to reduce environmental impact. Steel slag (SS), a by-product of steel production, shows potential as an alternative for natural aggregates (NA) in concrete. This study investigates the use of coarse SS aggregate (CSSA) and fine SS aggregate (FSSA) as partial replacements for natural coarse aggregate (NCA) and natural fine aggregate (NFA), respectively. Replacement levels for NCA were increased by 10% increments, ranging from 10 to 100%, while NFA was substituted at 10, 20, 30, 40, and 50% levels. The physical and chemical properties of the materials were analyzed to confirm their suitability in concrete. Concrete specimens were tested for compressive strength (CS), split tensile strength (STS), flexural strength (FS), water absorption (WA), and water permeability (WP). Concrete with 50% NCA and 20% NFA replaced by CSSA and FSSA, respectively, exhibited optimal results, with improvements in CS, STS, FS, and durability properties. These findings demonstrate that SS can effectively serve as an AA, resulting in more sustainable construction practices by reducing the use of NA and managing steel industry waste. This research underlines the importance of using industrial by-products in construction to align with global sustainability goals. The optimal replacement levels identified offer practical guidance for incorporating SS into concrete, supporting resource conservation, and reducing the environmental footprint of construction materials.