The synchronization of neuronal assemblies during cortical UP states has been implicated in computational and homeostatic processes, but the mechanisms by which this occurs remain unknown. To investigate potential roles of astrocytes in synchronizing cortical circuits, we electrically activated astrocytes while monitoring the activity of the surrounding network with electrophysiological recordings and calcium imaging. Stimulating a single astrocyte activates other astrocytes in the local circuit and can trigger UP state synchronizations of neighboring neurons. Moreover, interfering with astrocytic activity with intracellular injections of a calcium chelator into individual astrocytes inhibits spontaneous and stimulated UP states. Finally, both astrocytic activity and neuronal UP states are regulated by purinergic signaling in the circuit. These results demonstrate that astroglia can play a causal role in regulating the synchronized activation of neuronal ensembles.