Brain postnatal development is characterized by critical periods of experience-dependent remodeling of neuronal circuits. Failure to end these periods results in neurodevelopmental disorders. The cellular processes defining critical period timing remain unclear. Astrocyte processes, as structural and signaling partners of individual synapses, regulate neurotransmission and plasticity. Here we show in the mouse visual cortex that astrocytes control critical period closure. We uncover the underlying pathway, which involves astrocytic regulation of the extracellular matrix allowing interneuron maturation. Thus astrocytes not only influence activity of single synapses, but are also key elements in the experience-dependent wiring of brain circuits.