Abstract Alternative splicing–triggered nonsense-mediated mRNA decay (AS-NMD) critically regulates gene expression, but the extent to which neuronal genes are regulated by AS-NMD remains understudied. Here, we identified over 3,000 developmentally regulated AS-NMD exons in mouse and human brains and validated them in cultured neurons. AS-NMD suppresses synaptic genes during brain development and differentially regulates more than 200 causal genes for neurodevelopmental disorders (NDDs).