The regulated trafficking of neurotransmitter receptors at synapses is critical for synaptic function and plasticity. However, the molecular machinery that controls active transport of receptors into synapses is largely unknown. We found that, in rat hippocampus, the insertion of AMPA receptors (AMPARs) into spines during synaptic plasticity requires a specific motor protein, which we identified as myosin Va. We found that myosin Va associates with AMPARs through its cargo binding domain. This interaction was enhanced by active, GTP-bound Rab11, which is also transported by the motor protein. Myosin Va mediated the CaMKII-triggered translocation of GluR1 receptors from the dendritic shaft into spines, but it was not required for constitutive GluR2 trafficking. Accordingly, myosin Va was specifically required for long-term potentiation, but not for basal synaptic transmission. In summary, we identified the specific motor protein and organelle acceptor that catalyze the directional transport of AMPARs into spines during activity-dependent synaptic plasticity.