The C9orf72 mutation was identified as the most frequent genetic cause of frontotemporal dementia (FTD). In light of multiple reports of predominant psychiatric presentations of FTD secondary to C9orf72 mutation, the American Neuropsychiatric Association Committee on Research reviewed all studies on psychiatric aspects of this mutation to identify clinically relevant features for diagnosis. The most common psychiatric presentation is psychosis (21%-56%), with delusions, and/or multimodal hallucinations. Other presentations include late-onset mania and depression with cognitive impairment or catatonia. However, the frequency of C9orf72 mutations is low in typical schizophrenia or bipolar disorders (<0.1%). The authors provide clinical guidance on diagnosis and genetic testing.