Large quantities of waste glass are generated yearly. Glass powder is a non-cementitious, fine recycled glass product that is rich in silica. Using glass powder to stabilize the swelling of steel slag can be a sustainable geoenvironmental solution as it would eliminate the use of pozzolans and other cementitious materials. This study investigates the feasibility of using glass powder for stabilizing the long-term swelling strains of steel slag. For this purpose, basic oxygen furnace steel slag—glass powder mixtures were prepared using 5, 10, and 20% glass powder replacement ratios. Compaction and long-term swelling test were performed on all mixtures. After the completion of swelling tests, samples were investigated under a scanning electron microscope to determine the expansion products and to evaluate cementitious gel forming mechanisms. The results of this study showed that 10 and 20% glass powder replacement ratios resulted in mixtures with similar unit weight values, but those higher than that of the basic oxygen furnace steel slag. This indicated that at about 20% glass powder replacement ratio, the optimum fine percentage of the mixture was attained. The long-term swelling test results indicated higher swelling rates for the basic oxygen furnace steel slag-glass powder mixtures than those obtained for basic oxygen furnace steel slag. These higher, exacerbated swelling rates indicated the generation of unstable precipitates and gel forms with a low Ca/Si ratio. This study showed that glass powder replacement was not effective in alleviating the swelling response of the basic oxygen furnace-glass powder mixtures. This adverse effect on the swelling response of these mixtures was attributed to the sodium content that led to unstable gel-forming mechanisms.