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Rare loss-of-function variants in SRRM2, which encodes a nuclear speckle scaffold and splicing factor, are associated with schizophrenia and neurodevelopmental disorders. How SRRM2 haploinsufficiency disrupts brain function is unknown. We find that Srrm2 mice exhibit (1) large-scale changes in gene expression in neuronal and glial cells, affecting DNA-binding-, synapse-, translation-, mitochondria-related pathways across multiple brain regions; (2) alterations in splicing and/or abundance of multiple postsynaptic proteins, including reduction of the gamma isoform of SynGAP and elevation of its interactor, Agap3; and (3) reduced oligodendrocyte proportions, particularly in the striatum, accompanied by decreased expression of myelin-related mRNAs and proteins. Human induced pluripotent stem cell (iPSC)-derived neurons deficient in SRRM2 display conserved AGAP3 splicing defects. Behaviorally, Srrm2 mice have reduced locomotor activity and impaired startle responses, and electroencephalogram (EEG) recordings reveal reduced sleep spindles resembling humans with schizophrenia. Our findings identify specific synaptic changes, splicing dysregulation, and impaired myelination as mechanisms linking SRRM2 haploinsufficiency to neuropsychiatric disease.