There is an emerging interest in the utilization of low-cost industrial wastes in sustainable pollution control. This study investigated the use of water-quenched blast furnace slag modified by cetyltrimethyl ammonium bromide as a low-cost adsorbent for the removal of reactive brilliant red X-3B from an aqueous solution. The results showed that the removal efficiency increased with an increasing slag dosage and an appropriate dosage for adsorption was 8 g/L. The adsorption efficiency of X-3B onto modified slag decreased when the pH increased from 2 to 12. The amount of dye uptake per unit of modified slag also increased with an increase in ionic strength, contact time, temperature, and initial dye concentration. The Freundlich isotherm model showed a good fit to the equilibrium adsorption data. Thermodynamic studies suggested the spontaneous and endothermic nature of the adsorption process. The adsorption kinetics followed the pseudo-second-order model and the rate constant increased with increasing temperature. These results can help understand the migration patterns of dye molecules at a modified slag-water interface. It also has important implications for developing wastewater treatment techniques using low-cost adsorbents.