N-methyl-D-aspartate receptor (NMDAR) is a glutamate-gated ion channel permeable to calcium that plays critical roles in synaptic transmission, synaptogenesis, and plasticity. Recent sequencing data in neurological and psychiatric patients reveal numerous variants in genes encoding NMDAR subunits. In our study, we have created and evaluated a transgenic mouse line carrying a frameshift variant (Grin2b L825FfsX4) resulting in the deletion of about half of the GluN2B subunit. Our methods included patch-clamp recordings from hippocampal neurons in both wild-type (Grin2bWT/WT) and transgenic (Grin2bWT/L825FfsX4) mice, immunochemical analyses, and behavioral tests.The whole-cell NMDAR currents were reduced in Grin2bWT/L825FfsX4 neurons compared to Grin2bWT/WT mice. Additionally, sensitivity to ifenprodil, a GluN2B antagonist, decreased in Grin2bWT/L825FfsX4 neurons. While the peak NMDAR-evoked excitatory postsynaptic currents (eEPSC) amplitude at synapses remained unchanged, deactivation was significantly faster in Grin2bWT/L825FfsX4 neurons. These findings suggest a reduced functional contribution of GluN2B subunits to synaptic NMDAR currents in hippocampal neurones from Grin2bWT/L825FfsX4 mice. Analysis of GluN2B(L825FfsX4) subunit surface expression and synaptic delivery indicated changes compared to wild type, confirming altered delivery of the mutant subunit to synaptic sites. Behavioral testing revealed sex-dependent differences in activity levels, sensorimotor gating, cognition, and aggression.Our multidisciplinary approach provides a clinically relevant model for studying the implications of GluN2B variants in the pathophysiology of neurodevelopmental disease. The observed functional and pharmacological changes observed suggest a synaptic-level mechanism by which the GluN2B(L825FfsX4) variant may contribute to the neurodevelopmental pathology in carriers.