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Epileptic encephalopathies comprise a clinically and genetically heterogenous group of disorders characterized by global developmental delay and ongoing seizure activity. It is generally considered that seizure activity is the primary pathology and leads to altered cognitive function. However, epileptic encephalopathy can also result from primary defects in neurodevelopment that lead to seizures. Here we examine the symptomology of individuals with epileptic encephalopathy resulting from biallelic pathogenic variants in DENND5A, encoding a protein involved in intracellular membrane trafficking. We established a cohort of 15 individuals from 14 families with 21 unique variants in DENND5A and analyzed phenotypic surveys answered by their treating clinicians. We obtained cells derived from several cohort members and examined DENND5A expression and stability, and we examined the role of DENND5A in symmetric cell division capability. Finally, we generated a mouse line expressing a homozygous mutation found in the cohort and analyzed brain morphology in vivo using a 7 Tesla magnetic resonance imaging system. Our cohort study reveals that global developmental delay, gross abnormalities in brain development including ventriculomegaly and corpus callosum dysgenesis, seizures, microcephaly and hypotonia are found in the majority of the cohort. Patient-derived neural precursor cells from induced pluripotent stem cells, as well as patient-derived lymphoblasts, display loss of DENND5A protein and exhibit problems with symmetric cell division, an essential process in early neural development. The introduction of a homozygous human point mutation into the conserved sequence in mouse leads to an animal with reduced brain volume and widespread regional brain dysgenesis and ventriculomegaly, phenotypes shared with the human cohort. Taken together, our multi-dimensional study establishes DENND5A as a critical gene during neurodevelopment and that biallelic loss of function variants, including missense and truncating variants as well as intronic splice site variants, result in a developmental epileptic encephalopathy.