Gambling disorder has recently been recognized as a prototype 'behavioral addiction' by virtue of its inclusion in the DSM-5 category of 'Substance-Related and Addictive Disorders.' Despite its newly acquired status and prevalence rate of 1-3 % globally, relatively little is known regarding the neurobiology of this disorder. The aim of this study was to explore cortical morphometry in untreated gambling disorder, for the first time. Subjects with gambling disorder (N = 16) free from current psychotropic medication or psychiatric comorbidities, and healthy controls (N = 17), were entered into the study and undertook magnetic resonance imaging (3T MRI). Cortical thickness was quantified using automated segmentation techniques (FreeSurfer), and group differences were identified using permutation cluster analysis, with stringent correction for multiple comparisons. Gambling disorder was associated with significant reductions (average 15.8-19.9 %) in cortical thickness, versus controls, predominantly in right frontal cortical regions. Pronounced right frontal morphometric brain abnormalities occur in gambling disorder, supporting neurobiological overlap with substance disorders and its recent reclassification as a behavioral addiction. Future work should explore the trait versus state nature of the findings and whether similarities exist with other not-yet-reclassified putative behavioral addictions.