11C-BU99008 is a novel positron emission tomography (PET) tracer that enables selective imaging of astrocyte reactivity in vivo. To explore astrocyte reactivity associated with Alzheimer’s disease, 11 older, cognitively impaired (CI) subjects and 9 age-matched healthy controls (HC) underwent 3T magnetic resonance imaging (MRI), 18F-florbetaben and 11C-BU99008 PET. The 8 amyloid (Aβ)-positive CI subjects had higher 11C-BU99008 uptake relative to HC across the whole brain, but particularly in frontal, temporal, medial temporal and occipital lobes. Biological parametric mapping demonstrated a positive voxel-wise neuroanatomical correlation between 11C-BU99008 and 18F-florbetaben. Autoradiography using 3H-BU99008 with post-mortem Alzheimer’s brains confirmed through visual assessment that increased 3H-BU99008 binding localised with the astrocyte protein glial fibrillary acid protein and was not displaced by PiB or florbetaben. This proof-of-concept study provides direct evidence that 11C-BU99008 can measure in vivo astrocyte reactivity in people with late-life cognitive impairment and Alzheimer’s disease. Our results confirm that increased astrocyte reactivity is found particularly in cortical regions with high Aβ load. Future studies now can explore how clinical expression of disease varies with astrocyte reactivity.