Flow restrictors or rain blockers are devices installed at inlets or manholes to limit storm runoff entering a Combined Sewer System (CSS). One type of these structures is the vortex flow restrictor (VFR). Although a large number of this type of restrictor have been installed in CSSs in urban areas, the authors are not aware of any systematic study on the performance of the VFR in limiting peak flows. Herein the results of an experimental and numerical study of a VFR are presented. For the numerical study, a state-of-the-art three-dimensional Computational Fluid Dynamics (CFD) model was used. The numerical results for the discharge coefficient and rating curves show good agreement with the experimental observations. The results show that the flow discharge passing through a VFR can be reduced substantially compared to the case where no flow restrictor is in place. The results also show that the flow discharge depends strongly on the way the VFR is installed.