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Despite its remarkable effect on the activities of daily living, the precise mechanism underlying balance control after stroke remains to be elucidated. In this study, we investigated the cortical activation induced by postural perturbation in 20 patients with stroke using a 50-channel event-related functional near-infrared spectroscopy. A combination of brisk forward and backward movements of a platform without any prior cue was used as an external postural perturbation. Multi-participant analysis of oxygenated hemoglobin signals showed postural perturbation-related cortical activation in the prefrontal cortical areas in both hemispheres as well as the premotor and parietal association cortical areas in the unaffected hemisphere. Regression analysis using the individual Berg Balance Scale as the regressor showed a significant positive correlation between balance ability and the postural perturbation-related changes in oxygenated hemoglobin signals in the supplementary motor areas and prefrontal cortical areas in both hemispheres. Consistent with the previous findings in healthy participants, these findings suggest that the broad cortical network, including the prefrontal, premotor, supplementary motor, and parietal cortical areas in both hemispheres, was essential for balance control even in poststroke patients.