Inorganic salts (e.g., FeCl3 and FeSO4) are typically added to membrane bioreactor (MBR) for achieving improved total phosphorus (TP) removal and hydraulic performance. However, at the iron dose (e.g., Fe to P molar ratio of 1–4) required for effective TP removal, severe irreversible membrane fouling could become a significant concern. In this context, intermittent in-situ ozonation was provided for the first time directly in the membrane tank of Fe(II)-dosed membrane bioreactor, and its effect on membrane fouling mitigation was assessed and elucidated. The basic effluent quality remained unaffected, indicating that in-situ ozonation had no detrimental effects. In addition, concentration of foulants and the relative abundance of biofilm-forming microbes were comparable in both the Fe-MBR and Fe/O3-MBR. Importantly, as compared to the Fe-MBR, a 33% reduction in membrane fouling was achieved in the Fe/O3-MBR. The enhanced filterability of mixed liquor and the scrubbing of the membrane surface by ozone with the direct/indirect oxidation of the deposited foulants contributed to membrane fouling mitigation in the Fe/O3-MBR. Meanwhile, trans-membrane pressure profiles of MBRs followed cake-intermediate fouling mechanism. This study demonstrates that provision of intermittent in-situ ozonation is a simple and attractive approach to extend the operation of iron-dosed MBR.