For vortex settling basins (VSBs) installed with a deflector, perforation is an effective retrofit to reduce the self‐weight of the deflector and sediment deposition on it. The current study investigated experimentally the performance of VSBs the deflector of which was perforated at different locations with various opening ratios. The results showed that perforating the outside overflow area of the deflector was the optimum for reducing sediment deposition. With an opening ratio of 8.67–13% in the outside overflow area of the deflector, the VSB exhibited similar sediment removal efficiency to the original design without any openings on the deflector. The current study provided the design optimization for deflector perforation in VSBs.