By　directly　harming　cancer　cells,　radiotherapy　(RT)　is　a　crucial　therapeutic　approach　for　the　treatment　of　cancers.　However,　the　efficacy　of　RT　is　reduced　by　the　limited　accumulation　and　short　retention　time　of　the　radiosensitizer　in　the　tumor.　Herein,　we　developed　hypoxia-triggered　in　situ　aggregation　of　nanogapped　gold　nanospheres　(AuNNP@PAA/NIC　NPs)　within　the　tumor,　resulting　in　second　near-infrared　window　(NIR-II)　photoacoustic　(PA)　imaging　and　enhanced　radiosensitization.　AuNNP@PAA/NIC　NPs　demonstrated　increased　accumulation　and　retention　in　hypoxic　tumors,　mainly　due　to　the　hypoxia-triggered　aggregation.　After　aggregation　of　AuNNP@PAA/NIC　NPs,　the　absorption　of　the　system　extended　from　visible　light　to　NIR-II　light　owing　to　the　plasmon　coupling　effects　between　adjacent　nanoparticles.　Compared　to　the　normoxic　tumor,　the　PA　intensity　at　1200　nm　in　the　hypoxic　tumor　increased　from　0.42　to　1.88　at　24　h　postintravenous　injection　of　AuNNP@PAA/NIC　NPs,　leading　to　an　increase　of　4.5　times.　This　indicated　that　the　hypoxic　microenvironment　in　the　tumor　successfully　triggered　the　in　situ　aggregation　of　AuNNP@PAA/NIC　NPs.　The　in　vivo　radiotherapeutic　effect　demonstrated　that　this　hypoxia-triggered　in　situ　aggregation　of　radiosensitizers　significantly　enhanced　radiosensitization　and　thus　resulted　in　superior　cancer　radiotherapeutic　outcomes.　©　2023　American　Chemical　Society.