PEGylated polymer micelles containing 4-nitro-3-trifluoromethylphenyl units within the core moiety were prepared, and their phototriggered nitric oxide (NO)-generating ability was confirmed by electron spin resonance (ESR) spin-trapping and the Griess method. These micelles were found to be capable of delivering exogenous NO into tumor cells in a photocontrolled manner and showed an NO-mediated antitumor effect, indicating the usability of this molecular system in NO-based tumor therapy.