<p>A better understanding of the degradation mechanism of epoxy resins is crucial for the design and fabrication of materials with long-term durability. However, the understanding of this process has remained limited because the contributions of both heat and water must be considered simultaneously. In this study, we investigated the degradation mechanism of an amine-cured epoxy resin under hygrothermal conditions. Although the glass transition temperature (<i>T</i>_g) decreased with increasing hygrothermal aging time, this decrease was independent of the amount of sorbed water, indicating that plasticization was not the dominant factor. Instead, the reduction in the <i>T</i>_g was attributed to a decrease in the cross-linking density arising from C−O bond scission. Importantly, this scission was not based on acid-catalyzed cleavage but on radical-mediated cleavage. This process was induced by thermal decomposition followed by hydrogen donation from sorbed water. The insights obtained here provide molecular-level guidelines for designing epoxy resins with increased durability in hygrothermal environments.</p>