<p>Steel slag, a solid waste of the steel industry, is characterized by high levels of free calcium and magnesium oxides, and significant heavy metal content. Its disposal in landfills not only results in the loss of valuable resources but also poses serious environmental pollution concerns. Over the past few decades, significant attention has been directed toward utilizing steel slag in highway applications. This study offers a state-of-the-art review of research conducted between 1990 and 2025 on the use of steel slag as an alternative aggregate in asphalt mixture production. The paper reviews the conventional characteristics of steel slag covering its physical, chemical, and microscopic properties and examines how these influence the performance of steel slag–incorporated bituminous mixes. Performance aspects considered include stability, indirect tensile strength, rutting, fatigue, resilient modulus, moisture susceptibility, skid resistance, ravelling resistance, and low-temperature behaviour, based on findings from existing literature. The paper also reviews the impact of long-term aging on the engineering properties of steel slag mixes. Additionally, various treatment methods applied to steel slag aimed at enhancing the engineering performance of steel slag based asphalt mixtures also discussed. The paper concludes with a discussion on the global warming potential, cost analysis, and field performance of steel slag asphalt mixtures. Based on the comprehensive literature review, the optimal proportions of steel slag for producing asphalt mixture have been identified and presented. In addition, this paper highlights the existing gaps that require further scientific investigation and field validation, while also outlining potential directions for future research.</p>