Based on previous reports, UV/sulfite process is generally used as an advanced reduction process (ARP) since eaq− and/or ∙H, both with strong reduction potential, could be substantially generated herein. Very recently, the combination of UV and sulfite as an advanced oxidation process (AOP) or an oxidation-reduction coupling process has attracted increasing interest due to the yield of SO4∙− and/or HO∙. Herein, the application of UV/sulfite as an ARP and AOP (or oxidation-reduction coupling process) during water and wastewater treatments is reviewed respectively. (1) In the absence of O2, UV/sulfite works as an ARP. The generation mechanism of reactive reduction species and various contaminants removal (including degradation kinetics and efficiency, decomposition mechanisms, effects of some factors, etc.) is summarized in detail and systematically. Moreover, both the application of different types of UV lights and the economic evaluation are summarized systematically. (2) In the presence of O2, UV/sulfite could be used as an AOP or oxidation-reduction coupling process. The generation mechanism of reactive oxidation species and influencing factors is also presented in detail. Moreover, two ways (including homogeneous and heterogeneous activation) used to enhance the UV/sulfite oxidation potential are summarized respectively. Moreover, several knowledge gaps and research needs for further research are proposed. Overall, this review provides an overview for in-depth understanding of UV/sulfite as an ARP or AOP (oxidation-reduction coupling process) during water and wastewater treatments. Display omitted •The performance of UV/sulfite as an ARP was discussed in detail.•The application of various UV lights during UV/sulfite as an ARP was reviewed.•The energy evaluation of UV/sulfite as an ARP was summarized systematically.•UV/sulfite as an AOP or oxidation-reduction coupling process was presented.