Altered neuronal excitability is one of the hallmarks of fragile X syndrome (FXS), but the mechanisms underlying this critical neuronal dysfunction are poorly understood. Here, we find that pyramidal cells in the entorhinal cortex of Fmr1 KO mice, an established FXS mouse model, display a decreased AP threshold and increased neuronal excitability. The AP threshold changes in Fmr1 KO mice are caused by increased persistent sodium current (INaP). Our results indicate that this abnormal INaP in Fmr1 KO animals is mediated by increased mGluR5-PLC-PKC (metabotropic glutamate receptor 5/phospholipase C/protein kinase C) signaling. These findings identify Na+ channel dysregulation as a major cause of neuronal hyperexcitability in cortical FXS neurons and uncover a mechanism by which abnormal mGluR5 signaling causes neuronal hyperexcitability in a FXS mouse model. Display omitted •AP threshold is decreased in EC layer III pyramidal neurons of Fmr1 KO mice•AP threshold changes are caused by increased INaP in Fmr1 KO mice•Abnormal threshold is mediated by increased mGluR5-PLC-PKC pathway signaling Deng and Klyachko demonstrate that hyperexcitability of cortical excitatory neurons in a mouse model of fragile X syndrome is caused by a reduced action potential threshold. This excitability defect results from dysregulation of sodium channels by an exaggerated mGluR5-PLC-PKC pathway signaling.