Next-generation sequencing has been invaluable in the elucidation of the genetic etiology of many subtypes of intellectual disability in recent years. Here, using exome sequencing and whole genome sequencing, three de novo truncating mutations were identified in Wiskott- Aldrich syndrome protein family member 1 (WASF1) in five unrelated individuals with moderate to profound intellectual disability with autistic features and seizures. WASF1, also known as WAVE1, is part of the WAVE complex and acts as a mediator between Rac-GTPase and actin to induce actin polymerization. The three mutations connected using Matchmaker Exchange were c.1516C>T [p.Arg506Ter], which occurs in three unrelated individuals, c.1558C>T [p.Gln520Ter], and c.1482delinsGCCAGG [p.Ile494MetfsTer23]. All three variants are predicted to partially or fully disrupt the C-terminal actin-binding WCA domain. Functional studies using fibroblast cells from two affected individuals with the c.1516C>T mutation showed a truncated WASF1/WAVE1 protein and a defect in actin remodeling. This study provides evidence that de novo heterozygous mutations in WASF1 cause a rare form of intellectual disability.