BACKGROUNDS Adjustment disorder is a frequent mental illness that occurs under various stressful situations. Whereas adjustment disorder has distinct clinical manifestations and diagnostic entity, few studies have investigated its underlying neural substrate. This study aimed to identify brain structural abnormalities among patients with adjustment disorder. METHODS Twenty-five patients with adjustment disorder and 25 healthy controls participated in the study. Structural magnetic resonance imaging was performed, and a voxel-based morphometry was applied. Family-wise error-corrected p values for statistical analysis of comparative gray matters between patients with adjustment disorder and healthy controls were used. RESULTS Patients with adjustment disorder had decreased gray matter volume in the right medial frontal gyrus as compared to healthy controls. There were no brain regions that were decreased in the healthy controls as compared to patients with adjustment disorder. LIMITATIONS This study was a cross-sectional design. CONCLUSIONS Our results suggest that adjustment disorder arises from characteristic neural abnormalities, contrary to previous notions suggesting that adjustment disorder is a non-specific and/or residual diagnostic term. Moreover, future studies should examine the underlying neural substrates responsible for successful adaptation to unfamiliar and stressful situations.