Abstract: In China, the sulfate levels in acid mine drainage (AMD) are generally high. Developing an effective method to eliminate sulfate from wastewater is imperative for AMD treatment. Considering the promising prospect of treating AMD with converter steel slag, we analyze the removal effect and mechanism of sulfate in AMD treatment with steel slag by a single-factor experimental method. The adsorption amount and removal rate are determined using an ultraviolet spectrophotometer. The findings reveal that the removal efficiency of sulfate was influenced by the particle size of steel slag, pH of wastewater, solid–liquid ratio, and sulfate concentration. When the particle size of steel slag is less than 75 μm, the pH of the system is 2, the solid–liquid ratio is 70 g·L⁻¹, and the initial sulfate concentration is 2000 mg·L⁻¹; moreover, the removal rate of sulfate is the highest, which is 79.15% and the adsorption capacity is 36.79 mg·g⁻¹. By linearly fitting the adsorption capacity data to the Langmuir and Freundlich equations, the adsorption process of sulfate on the steel slag surface is more in line with the Freundlich adsorption model. Furthermore, thermodynamic calculations demonstrate that the Gibbs free energy change (∆G) of adsorption is negative and enthalpy change (∆H) and entropy change (∆S) are both positive, which demonstrates that the process of sulfate adsorption on steel slag is spontaneous, entropy-driven, and endothermic chemical adsorption. Moreover, when the adsorption temperature is between 301 and 321 K, the adsorption capacity increases with increasing adsorption time and temperature, and when the adsorption time reaches 1320 min, the increasing trend of adsorption capacity slows down. Kinetic calculation results indicate that the equilibrium adsorption amount calculated using the secondary reaction rate equation is similar to the experimental equilibrium adsorption capacity, suggesting that the adsorption of sulfate on the steel slag surface conforms to the secondary adsorption kinetic model. Sulfate removal is mainly controlled by chemical processes, and material transport is not the main controlling step. Based on the intraparticle diffusion fitting results, the adsorption process goes through three stages: external rapid adsorption, internal gradual diffusion, and finally equilibrium. X-ray diffraction of steel slag before and after adsorption exhibits that sulfate existed on the steel slag surface in the form of gypsum, which was corroborated by Fourier-transform infrared spectroscopy analysis and scanning electron microscopy characterization. In conclusion, the reaction between steel slag and sulfate is dominated by multilayer chemical adsorption combined with chemical precipitation, electrostatic adsorption, and surface complexation.