Brain water homeostasis provides not only physical protection, but also determines the diffusion of chemical molecules key for information processing and metabolic stability. As a major type of glial cell in the brain parenchyma, astrocytes are the dominant cell type expressing aquaporin water channels. However, how astrocyte aquaporin contributes to brain water homeostasis remains to be understood. We report here that astrocyte aquaporin 4 (AQP4) mediates a tonic water efflux in basal conditions. Acute inhibition of astrocyte AQP4 leads to intracellular water accumulation as optically resolved by fluorescence-translated imaging in acute brain slices, and in vivo by fiber photometry in moving mice. We then show that the tonic aquaporin water efflux maintains astrocyte volume equilibrium, astrocyte and neuron Ca 2+ signaling, and extracellular space remodeling during optogenetically induced cortical spreading depression. Using diffusion-weighted magnetic resonance imaging (DW-MRI), we observed that in vivo inhibition of AQP4 water efflux heterogeneously disturbs brain water homeostasis in a region-dependent manner. Our data suggest that astrocyte aquaporin, though bidirectional in nature, mediates a tonic water outflow to sustain cellular and environmental equilibrium in brain parenchyma. Significance statement Our brain is immersed, thus protected, in a water environment. It ensures intra– and extracellular molecular diffusion, which is vital for brain function and health. Brain water homeostasis is maintained by dynamic water transport between different cell types. Astrocytes are a main type of glial cell widely distributed in brain parenchyma, and are also the primary cell type expressing the bidirectional aquaporin water channel. Here we show that in basal conditions, aquaporin channel mediates a tonic water efflux from astrocytes. This mechanism maintains astrocyte volume stability, activity-gated brain parenchyma remodeling and brain water homeostasis. Our finding sheds light on how astrocytes regulate water states in the brain, and will help to understand brain homeostasis in specific life context.