In our case report, a 29-year-old male patient with a known history of schizophrenia presented with altered mental status and catatonia and was found to have an enlarged (21mm) cavum septum pellucidum (CSP) on magnetic resonance imaging (MRI). He was subsequently treated with escitalopram, olanzapine, methylphenidate, lorazepam, and eight electroconvulsive therapy (ECT) treatments during his hospital course, after which his catatonia improved. We compared this to other cases in which a large CSP was identified and discussed the possibility of increased susceptibility to psychosis, specifically catatonia, which might be associated with this developmental anomaly.