Abstract: The utilization of high-value-added metallurgical solid waste, such as the use of an inexpensive specialty-steel slag as a rubber functional filler, is an important sustainable development strategy. In this study, we prepared specialty-steel slag-based rubber composites from specialty-steel slag, carbon black, an accelerator, sulfur, zinc oxide, stearic acid, and compound rubber. Then we conducted tests to determine the internal exposure index, external exposure index, stability, tensile strength, tear strength, elongation at break, shore A hardness, limiting oxygen index, burnout time, leaching concentration of heavy metals, mineral composition, particle size distribution, heat conductivity coefficient, pore structure, chemical composition, microstructure, and thermal stability of the composites. We also studied the feasibility and environmental risk associated with using specialty-steel slag as a rubber functional filler. The results show that the mineral composition of the specialty-steel slag includes Ca₂SiO₄, Ca₃Al₆Si₂O₁₆, (Fe, Mn)₂SiO₄, Ca₃Al₂(SiO₄)₃, Na₂TiSiO₅, CuMn₆SiO₁₂, Na₂SiO₅, Pb₃Ta₂O₈, Pb₃SiO₇, and other solid metal melts. This slag also has a good particle size distribution, and its safety and stability meet the requirements of relevant national standards. When the content of the specialty-steel slag in specialty-steel slag-based rubber composites ranges between 20%–40%, these composites have a tensile strength ranging from 20.0–21.5 MPa, a tear strength of 45.2–48.6 kN·m⁻¹, an elongation at break value of 475%–501%, a shore A hardness of 63.5–65.3, a limiting oxygen index of 18.5–18.6, a burnout time of 264–292 s, and a heat conductivity coefficient of 0.15–0.17 W·m⁻¹·K⁻¹. The main heavy-metal oxides in the specialty-steel slag are identified as Cr₂O₃, PbO, and CuO, which mainly exist as stable solid metals. In addition, the leaching concentration of the heavy metals, such as Cu, Zn, Cd, Pb, Cr, Ba, Ni, and As, from the specialty-steel slag-based rubber composites is much lower than the limit value of the hazardous-waste identification standards. Therefore, specialty-steel slag is safe and feasible for use as a rubber functional filler.