Phosphorus (P) removal in constructed wetlands (CWs) is often low unless special substrates with high sorption capacities are used. However, the use of special substrates in vertical flow (VF) CWs has not been proved to enhance P sorption. Thus, two VF wetlands were designed to evaluate the potential for enhanced P removal from polluted urban river water, one with slag as substrate and the other as a control with gravel as substrate. Findings from batch experiments showed P sorption capacities of 3.15 gP/kg and 0.81 gP/kg, respectively, for steel slag and gravel. Different organic matter fractions played different roles in P sorption, the effects of which were significant only at high concentrations. Over a 220 days' operation, the VF-slag removed 76.0% of the influent total phosphorus (TP) at 0.159 g/m2·d and PO4-P of 70.9% at 0.063 g/m2·d, whereas the VF-gravel removed 65.0% at 0.136 g/m2·d and 48.6% at 0.040 g/m2·d, respectively. Therefore, the merit of using a steel slag substrate in VF wetlands can be significant for the removal of PO4-P.