Coastal hypoxia often develops in stratified estuaries, where river plume expansion and bottom salinity intrusion coexist. However, how surface plume fronts alter bottom hypoxia is not straightforward. In situ observations in the Pearl River Estuary showed that bottom hypoxia can be alleviated by frontal subductions. We observed a bottom-attached subductionevent during the passage of an active front, driven by surface convergence with weak mixing. This subduction effectively transports surface dissolved oxygen (DO)-rich waters to the bottom, increasing bottom DO concentrations. While this downward transport is often suppressed by a mid-depth barrier layer in the three-layer stratified flow, it can move along the sloping isopycnal during the slack tide. Our findings reveal a mechanism that frontal processes can mitigate hypoxia via the subduction of surface DO-rich waters along tilted isopycnals during the slack tide, thereby helping improve the numerical modelling accuracy of the extent and degree of coastal hypoxia.