Phosphorus (P) pollution is a major contributor to eutrophication, while the accumulation of solid wastes such as steel slag and biomass without proper disposal poses significant environmental risks. To address these challenges, biochar/steel slag (BC/SS) composites were synthesized via the carbonation of BC and SS as a novel adsorbent to remove P from simulated wastewater. In this study, the carbonated BC/SS effectively captured P and reduced the pH and conductivity of the treated solutions. Different proportions of BC and SS were tested to optimize the adsorptive performance of the composites. Results show that BC10 (containing10 wt.% of BC) exhibited the highest adsorption capacity for P in the batch experiments. The adsorption process followed the pseudo-second-order kinetics and conformed closely to the Langmuir isotherm, with a maximum adsorption capacity (qm) of 5.36 (mg/g) in 48 h. Column experiments further confirmed the strong P removal efficiency of the composite. Material characterisations revealed that carbonation primarily led to the formation of calcium carbonate from Ca-species, and the addition of BC enhanced the carbonation reaction. This study highlights the potential of carbonated BC/SS composites as an effective adsorbent for P removal from wastewater, offering a sustainable method for the utilization of solid wastes.