Updated project metadata. Fragile X syndrome (FXS) is the most common monogenetic cause of inherited intellectual disability and autism in humans. One of the well-characterized molecular phenotypes of Fmr1 KO mice, a model of FXS, is increased translation of synaptic proteins. Although this upregulation stabilizes in the adulthood, abnormalities during the critical period of plasticity have long-term effects on circuit formation and synaptic properties. Using high-resolution quantitative proteomics of synaptoneurosomes isolated from the adult, developed brains of Fmr1 KO mice, we show differential abundance of proteins regulating postsynaptic receptor activity of glutamatergic synapse. This work includes proteomic dataset that was acquired and analyzed to quantify changes in the abundance of proteins in synaptoneurosomes (basal state, unstimulated and in vitro NMDA-R stimulated) isolated from Fmr1 KO mice and their wild-type littermates.