The cerebral cortex is built during embryonic neurogenesis, a period when excitatory neurons are generated from progenitors. Defects in neurogenesis can cause acute neurodevelopmental disorders, such as microcephaly (reduced brain size). Altered dosage of the 1q21.1 locus has been implicated in the etiology of neurodevelopmental phenotypes; however, the role of 1q21.1 genes in neurogenesis has remained elusive. Here, we show that haploinsufficiency forRbm8a, an exon junction complex (EJC) component within 1q21.1, causes severe microcephaly and defective neurogenesis in the mouse. At the onset of neurogenesis,Rbm8aregulates radial glia proliferation and prevents premature neuronal differentiation. ReducedRbm8alevels result in subsequent apoptosis of neurons, and to a lesser extent, radial glia. Hence, compared to control,Rbm8a-haploinsufficient brains have fewer progenitors and neurons, resulting in defective cortical lamination. To determine whether reciprocal dosage change ofRbm8aalters embryonic neurogenesis, we overexpressed humanRBM8Ain two animal models. Usingin uteroelectroporation of mouse neocortices as well as zebrafish models, we findRBM8Aoverexpression does not significantly perturb progenitor number or head size. Our findings demonstrate thatRbm8ais an essential neurogenesis regulator, and add to a growing literature highlighting roles for EJC components in cortical development and neurodevelopmental pathology. Our results indicate that disruption ofRBM8Amay contribute to neurodevelopmental phenotypes associated with proximal 1q21.1 microdeletions.