Synapses are asymmetric cellular adhesions that are critical for nervous system development and function, but the mechanisms that induce their formation are not well understood. We have previously identified thrombospondin as an astrocyte-secreted protein that promotes central nervous system (CNS) synaptogenesis. Here, we identify the neuronal thrombospondin receptor involved in CNS synapse formation as α2δ-1, the receptor for the anti-epileptic and analgesic drug gabapentin. We show that the VWF-A domain of α2δ-1 interacts with the epidermal growth factor-like repeats common to all thrombospondins. α2δ-1 overexpression increases synaptogenesis in vitro and in vivo and is required postsynaptically for thrombospondin- and astrocyte-induced synapse formation in vitro. Gabapentin antagonizes thrombospondin binding to α2δ-1 and powerfully inhibits excitatory synapse formation in vitro and in vivo. These findings identify α2δ-1 as a receptor involved in excitatory synapse formation and suggest that gabapentin may function therapeutically by blocking new synapse formation.