The p21-activated kinase (PAK) family of proteins function as key effectors of RHO family GTPases in mammalian cells to regulate many pathways including Ras/Raf/MEK/ERK and Wnt/β-catenin, amongst others. Here we report an individual with a novel autosomal dominant disorder characterized by severe regressive autism, intellectual disability, and epilepsy. Exome sequencing of the proband and her parents revealed a de novo variant in the PAK1 gene ([NM_001128620] c. 362C>T/p.Pro121Leu). Studies in patient cells showed a clear effect on PAK1 protein function, including altered phosphorylation of targets (JNK and ERK), decreased abundance of β-catenin, and concomitant altered expression downstream of these key regulators. Our findings add PAK1 to the list of PAK proteins and kinases which when mutated cause rare genetic diseases.