Oligophrenin-1 (OPHN1) encodes a Rho-GTPase-activating protein (Rho-GAP) whose loss of function has been associated with X-linked mental retardation (MR). The pathophysiological role of OPHN1, however, remains poorly understood. Here we show that OPHN1 through its Rho-GAP activity plays a critical role in the activity-dependent maturation and plasticity of excitatory synapses by controlling their structural and functional stability. Synaptic activity through NMDA receptor activation drives OPHN1 into dendritic spines, where it forms a complex with AMPA receptors, and selectively enhances AMPA-receptor-mediated synaptic transmission and spine size by stabilizing synaptic AMPA receptors. Consequently, decreased or defective OPHN1 signaling prevents glutamatergic synapse maturation and causes loss of synaptic structure, function, and plasticity. These results imply that normal activity-driven glutamatergic synapse development is impaired by perturbation of OPHN1 function. Thus, our findings link genetic deficits in OPHN1 to glutamatergic dysfunction and suggest that defects in early circuitry development are an important contributory factor to this form of MR.