Astrocyte Ca²⁺ dynamics control synaptic circuits and behavior, yet the underlying biology remains poorly understood. By combining volumetric high-resolution electron microscopy and two-photon Ca²⁺ imaging, we characterize astrocyte leaflets that interface with synapses. These convoluted structures with ≤250 nm diameter originate from astrocytic shafts or cell bodies, contain minuscule endoplasmic reticulum saccules expressing IP₃ receptors but not mitochondria, and are often interconnected via gap junctions forming domains with cytosolic continuity. Leaflets enwrap 90% of synapses in clusters and only 10% individually. By fast imaging of astrocyte peripheral microvolumes, we identify leaflet-specific Ca²⁺ events that were synaptically induced, IP₃ R1-mediated, and often displayed separate originations merging into large, long-lasting Ca²⁺ elevations. Using combined axon-leaflet Ca²⁺ imaging, we show that these complex events reflect integration of incoming inputs from different neurons. The astrocyte leaflet organization may thus coordinate, via Ca²⁺ signals, multiple synapses and circuits active at different spatiotemporal scales, executing computations distinct from neurons.