Hypofunction of the N-methyl D-aspartate subtype of glutamate receptor (NMDAR) is hypothesized to be a mechanism underlying cognitive dysfunction in individuals with schizophrenia. For the schizophrenia-linked genes NRG1 and ERBB4, NMDAR hypofunction is thus considered a key detrimental consequence of the excessive NRG1-ErbB4 signaling found in people with schizophrenia. However, we show here that neuregulin 1β-ErbB4 (NRG1β-ErbB4) signaling does not cause general hypofunction of NMDARs. Rather, we find that, in the hippocampus and prefrontal cortex, NRG1β-ErbB4 signaling suppresses the enhancement of synaptic NMDAR currents by the nonreceptor tyrosine kinase Src. NRG1β-ErbB4 signaling prevented induction of long-term potentiation at hippocampal Schaffer collateral-CA1 synapses and suppressed Src-dependent enhancement of NMDAR responses during theta-burst stimulation. Moreover, NRG1β-ErbB4 signaling prevented theta burst-induced phosphorylation of GluN2B by inhibiting Src kinase activity. We propose that NRG1-ErbB4 signaling participates in cognitive dysfunction in schizophrenia by aberrantly suppressing Src-mediated enhancement of synaptic NMDAR function.