Cerebrovascular reactivity (CVR) is the response of cerebral blood vessels to vasoactive stimuli. Dampened CVR can precede and contribute to neuropathology. However, CVR assessments in glaucoma patients have been lacking at the whole-brain scale. Here, we applied relative CVR (rCVR) mapping using resting-state fMRI to investigate vascular reserve changes in glaucoma patients. Our results show that visual cortical rCVR decreases with severity and is coupled with clinical ophthalmic assessments. Interestingly, rCVR in both basal forebrain and hippocampus increase with severity indicating their involvements in glaucoma. Together, resting-state fMRI derived rCVR can potentially be used for studying, diagnosing and monitoring glaucoma.