The N-methyl-D-aspartate subtype of glutamate receptors (NMDAR) is a key regulator of brain plasticity encoding learning and memory. and In addition to glutamate, NMDAR activation requires the binding of the co-agonist D-serine. The beta-amyloid (Aß) peptide which accumulates in Alzheimer's disease (AD) through increased production and decreased clearance, affects the D-serine-dependent NMDAR activation in vitro, but whether this alteration would significantly contribute to mechanisms of AD-related pathophysiology and memory deficits remains controversial unclear. Herein, we report a decrease in the maximal pool of recruitable NMDAR and in the expression of long-term potentiation at CA3/CA1 synapses from hippocampal slices of 5xFAD mouse, an AD-related model with elevated Aß levels. These synaptic impairments develop from 1.5-2 months of age with the initial rise of Aß and is accompanied by a transient increase in D-serine levels. Deficits in working and spatial memories as well as cognitive flexibility then occurred in 10-12 months-old animals. Importantly, the synaptic deregulation and most of memory impairments are prevented in 5xFAD mice devoid of D-serine when the synthesis enzyme serine racemase is genetically deleted. Altogether, these results therefore provide for the first time in vivo evidence for the implication of D-serine in the early pathogenic signatures of AD driven by amyloidogenesis.