Autism spectrum disorder (ASD) and intellectual disability (ID) are neurodevelopmental diseases associated with various genetic mutations. Recent clinical studies report that chromosomal 12q24.31 microdeletions are associated with human ASD/ID. However, the causality and underlying mechanisms linking 12q24.31 microdeletions to ASD/ID pathogenesis remain undetermined. Here we show Kdm2b , one of the genes located in chromosomal 12q24.31, plays a critical role in maintaining neural stem cells (NSCs) in the developing mouse brain. Loss of the CxxC-ZF domain of KDM2B impairs its function in recruiting Polycomb repressive complex 1 (PRC1) to chromatin, resulting in de-repression of genes involved in cell apoptosis, cell cycle arrest, NSC premature senescence, and leading to the loss of NSC populations in the brain. Importantly, the Kdm2b mutation is sufficient to induce ASD/ID-like social and memory deficits in adult mice. Thus, our study reveals a critical role of an epigenetic factor KDM2B in normal brain development, a causality between the Kdm2b mutation and genesis of ASD/ID-like phenotypes in mice, and potential molecular mechanisms linking the function of KDM2B-PRC1 in transcriptional regulation and NSC senescence to the12q24.31 microdeletion-associated ASD/ID.