We evaluated the in vivo antioxidant activity of genistein in a mouse model of singlet oxygen-induced cerebral stroke. Cerebral stroke was induced in male BALB/c mice through extensive microvessel damage caused by photoactivated rose bengal dye. The photoactivation of the intravenously administered rose bengal was achieved by transcranial illumination with green light. Genistein was more active than its analogs and other antioxidants that were used as control agents. At a dose of 16 mg/kg genistein administered every 6 h from 24 h prior to irradiation until 24 h after irradiation, the average size of the cerebral lesion of genistein-treated mice was significantly smaller than that in control mice treated with the carrier DMSO (8.1 ± 1.0 mm2 compared to 14.6 ± 0.7 mm2, P < 0.001). Our findings provide experimental evidence that genistein could be useful for the prevention of cerebral stroke and other pathological conditions caused by reactive oxygen species.