Tetraploidy is a rare chromosomal abnormality that is typically lethal in utero, with limited data on its effects on brain development. A comprehensive neuropathologic study of a tetraploid newborn (92, XXYY) at a corrected gestational age of 30 weeks is presented. Findings included hippocampal hypoplasia, partial corpus callosum agenesis, and neuronal heterotopies. In addition, the cerebral cortex showed sparse neurons across all laminae, along with molecular and meningeal glioneuronal heterotopies. The periventricular region demonstrated dispersed germline neurons, and the cerebellum exhibited matrix cell heterotopies in the dentate nuclei and numerous migrating Purkinje cells in the white matter, indicating immaturity. These brain abnormalities may explain the severe developmental delays and intellectual impairment seen in surviving patients. Unlike other severely unbalanced chromosomal aberrations that typically result in major brain malformations, tetraploidy appears to have a less severe effect on brain development. This report expands the knowledge of brain abnormalities in tetraploidy.