Ferroptosis has appealing antitumor potential that is mainly based on the accumulation of lipid peroxide to a lethal level. The cytotoxic singlet oxygen (O) generated from nanoscale X-ray-induced photodynamic therapy (X-PDT) may facilitate glutathione (GSH) depletion and further activate ferroptosis. To realize combined X-PDT and ferroptosis, a nanocarrier (D-NP) was engineered with a hyperbranched copolymer with O-sensitive linkers, where both the photosensitizer (verteporfin) and ferroptosis inducer RAS-selective lethal small molecule 3 (RSL3) were encapsulated. Upon X-ray radiation, D-NP could produce a large amount of O for apoptosis. Subsequently, O triggered D-NP dissociation by cleavage of 1,2-bis(2-hydroxyethylthio)ethylene bonds to boost payload release and decrease levels of intracellular GSH thiol oxidation. Liberated RSL3 is a covalent inhibitor for glutathione peroxide 4 (GPX4), which is responsible for detoxifying lipid peroxides to lipid alcohols with GSH assistance, and both O-induced GSH depletion and GPX4 inactivation thereby produced ferroptotic cell death. Tumor growth inhibition in murine 4T1 tumor-bearing mice demonstrated that D-NP produced pronounced therapeutic efficiency where ferroptosis induction was supported by the GPX4 content and expression. This study highlights the contribution of O-sensitive nanocarriers for promoting the potency of combined X-PDT and ferroptosis.