Astrocyte-derived lactate, through the astrocyte–neuron lactate shuttle, fuels neuronal energy demands and acts as a signalling molecule promoting synaptic plasticity and memory consolidation. Lactate regulates neuronal excitability and expression of genes related to synaptic plasticity and neuroprotection, but the molecular mechanisms remain unclear. Using patch-clamp recordings in cultured cortical neurons we found that lactate enhances NMDA receptor currents (INMDAR), increasing their amplitude and decay time constant. Not reproduced by HCAR1 agonists, this modulation depends on monocarboxylate transporters and lactate dehydrogenase, requiring lactate entry, metabolic conversion to pyruvate and NADH formation within neurons. Disruption of intracellular calcium dynamics or inhibition of Ca2+/calmodulin-dependent protein kinase II (CaMKII) diminishes lactate's effects on INMDAR. Two redox-sensitive cysteine-containing sequences in the intracellular C-terminal domain of GluN2B subunit play a crucial role in the potentiation of NMDAR by lactate. Experiments in HEK cells demonstrate that functional CaMKII and GluN2B-containing NMDARs are necessary for lactate's effects. Mutations in GluN2B, that disrupt either CaMKII binding or cysteine-mediated redox regulation, abolish lactate's modulatory action. Immunoprecipitation experiments in neurons show that lactate promotes CaMKII-GluN2B association, which is critical for increasing INMDAR amplitude. Proximity ligation assays between GluN2B and PSD-95 reveal that lactate induces GluN2B accumulation in dendritic spines, an effect a CaMKII inhibitor prevents. These findings elucidate a pathway whereby lactate enhances NMDAR function through metabolic conversion and redox-sensitive interactions requiring CaMKII, linking astrocyte energy metabolism to synaptic modulation.