Satsuma mandarin (Citrus unshiu Marc.) is one of the globally cultivated citrus species. The bacterial pathogen, Xanthomonas citri pv. citri, which causes citrus canker, is typically managed using sodium hypochlorite-based disinfection in quarantine settings. We assessed whether gaseous chlorine dioxide (ClO₂) could be used for postharvest disinfection of Satsuma mandarins. A disinfection system for this purpose was developed using an 80L plastic container, a gaseous ClO₂-generating reagent, and a gas monitoring device. Under baseline conditions, gaseous ClO₂ concentration increased markedly from 0 to 15 min, then gradually increased to around 60 ppm at 60 min. However, gaseous ClO₂ levels were reduced by absorption or degradation due to contact with the plates and fruit. The system completely inhibited pathogen growth on all test plates and effectively disinfected the fruits after 1 h of exposure, with no significant effect on fruit quality. The disinfection efficacy of gaseous ClO₂ was comparable to that of sodium hypochlorite. The CT (concentration × time) values obtained in this study may serve as a useful parameter for scaling up the system under conditions that balance effective pathogen control with the preservation of fruit quality.