Neuronal migration defects, including pachygyria, are among the most severe developmental brain defects in humans. Here, we identify biallelic truncating mutations in CTNNA2, encoding alpha N-catenin, in patients with a distinct recessive form of pachygyria. CTNNA2 was expressed in human cerebral cortex, and its loss in neurons led to defects in neurite stability and migration. The alpha N-catenin paralog, alpha E-catenin, acts as a switch regulating the balance between beta-catenin and Arp2/3 actin filament activities(1). Loss of alpha N-catenin did not affect beta-catenin signaling, but recombinant alpha N-catenin interacted with purified actin and repressed ARP2/3 actin-branching activity. The actin-binding domain of alpha N-catenin or ARP2/3 inhibitors rescued the neuronal phenotype associated with CTNNA2 loss, suggesting ARP2/3 de-repression as a potential disease mechanism. Our findings identify CTNNA2 as the first catenin family member with biallelic mutations in humans, causing a new pachygyria syndrome linked to actin regulation, and uncover a key factor involved in ARP2/3 repression in neurons.