Wastewater contaminated with organic compounds is a serious problem; therefore, many catalysts, especially copper catalysts, have been developed to treat it and remove contaminants before discharge. However, such separation and reuse of these catalysts is often challenging. Steel slag (SS), a by-product of steel production, is produced in large quantities and requires careful disposal. Therefore, in this study, we developed a magnetically recyclable copper catalyst utilizing pre-treated magnetic steel slag (MSS) as a support. First, magnetic separation was carried out to remove calcium silicate impurities such as alite and belite in MSS up to five times, thus increasing the Fe content of the MSS. We synthesized the Cu catalyst supported by MSS (donated as Cu@MSS) and characterized the catalyst by various surface analysis techniques, showing the presence of CuO and CuCO3 nanoparticles on the MSS surface. In catalytic reduction tests of para-nitrophenol using sodium borohydride in the presence of Cu@MSS, the reaction was accelerated when using the five-times pre-treated MSS because of the removal of inhibitors such as calcium compounds, as well as the high content of iron oxides leading to a synergetic effect with metallic Cu in this study. In addition, we investigated the effects of various factors, including Cu loading, sodium borohydride concentration, and catalyst dosage, on the catalytic activity of Cu@MSS. The catalyst was found to be stable and reusable. In summary, these results suggest that treated SS can be used as a support material for copper catalysts for the treatment of contaminated wastewater and the easy separation and reuse of the catalyst.