Astrocytes control the formation and maturation of both excitatory and inhibitory synapses through secreted proteins. However, the specific astrocyte-secreted signals that induce inhibitory synaptogenesis have been unclear. This study identifies neurocan (NCAN) as an astrocyte-secreted protein that controls SST+ inhibitory synapse formation and function in the developing cerebral cortex. NCAN is cleaved into N- and C-terminal fragments, with the C-terminal fragment localizing to synapses and being necessary and sufficient for SST+ inhibitory synaptogenesis. Astrocyte-specific NCAN knockout mice exhibit reduced inhibitory synapse numbers and function, highlighting the importance of astrocyte-secreted molecules in controlling excitatory/inhibitory synaptic balance and brain connectivity. The findings also suggest that NCAN C-terminal may control distinct inhibitory synaptic circuits, such as SST+ synapses, which could have implications for neurological disorders and brain injury.Astrocytes control the formation and maturation of both excitatory and inhibitory synapses through secreted proteins. However, the specific astrocyte-secreted signals that induce inhibitory synaptogenesis have been unclear. This study identifies neurocan (NCAN) as an astrocyte-secreted protein that controls SST+ inhibitory synapse formation and function in the developing cerebral cortex. NCAN is cleaved into N- and C-terminal fragments, with the C-terminal fragment localizing to synapses and being necessary and sufficient for SST+ inhibitory synaptogenesis. Astrocyte-specific NCAN knockout mice exhibit reduced inhibitory synapse numbers and function, highlighting the importance of astrocyte-secreted molecules in controlling excitatory/inhibitory synaptic balance and brain connectivity. The findings also suggest that NCAN C-terminal may control distinct inhibitory synaptic circuits, such as SST+ synapses, which could have implications for neurological disorders and brain injury.