Spatially Synergistic Dual‐Confinement Enables Underwater‐Stable Room‐Temperature Phosphorescent Hydrogels

Organic room‐temperature phosphorescent (RTP) hydrogels are attractive as flexible luminescent materials for optoelectronic and biomedical applications, but their water‐rich and mechanically compliant nature makes it difficult to create microenvironments that simultaneously suppress nonradiative decay, resist external quenching, and maintain structural robustness. Here, we report an amphiphilic polymer chain‐driven in situ curing strategy that generates rigid luminescent epoxy microspheres uniformly within a PVA hydrogel matrix. Combined with annealing–reswelling‐induced matrix densification, this process creates a spatially synergistic dual‐confinement system, in which the inner epoxy domains immobilize phosphors while the outer dense PVA network helps impede the ingress of water and oxygen. The resulting hydrogels exhibit tunable multicolor RTP, an ultralong phosphorescence lifetime of up to 4342 ms, a daylight‐visible afterglow luminance of 87.9 cd m −2 , a time‐gated delayed‐emission quantum yield of 55.49%, and a tensile strength exceeding 10 MPa. Notably, the hydrogels retain a phosphorescence lifetime of 3172 ms after 170 days of water immersion. This work identifies spatially synergistic dual confinement as an effective route to jointly optimize phosphorescence efficiency, mechanical robustness, and long‐term underwater stability in soft luminescent hydrogels.