EphB receptor tyrosine kinases are enriched at synapses, suggesting that these receptors play a role in synapse formation or function. We find that EphrinB binding to EphB induces a direct interaction of EphB with NMDA-type glutamate receptors. This interaction occurs at the cell surface and is mediated by the extracellular regions of the two receptors, but does not require the kinase activity of EphB. The kinase activity of EphB may be important for subsequent steps in synapse formation, as perturbation of EphB tyrosine kinase activity affects the number of synaptic specializations that form in cultured neurons. These findings indicate that EphrinB activation of EphB promotes an association of EphB with NMDA receptors that may be critical for synapse development or function.