Despite increasing empirical interest in muscle dysmorphia (MD), a dearth of research has assessed this construct in sexual minority populations. In particular, the psychometric properties of one of the most widely used measures of MD symptoms-the Muscle Dysmorphic Disorder Inventory (MDDI)-have not been evaluated in sexual minority populations despite emerging evidence suggesting differential risk for MD symptoms across sexual orientation groups. In this study, we assessed the psychometric properties of the MDDI in a sample of 715 cisgender gay men and 404 cisgender lesbian women ages 18-50 years who participated in a large-scale national longitudinal cohort study of sexual and gender minority adults. The factor structure of the MDDI was examined in each sample using a two-step, split-sample exploratory and confirmatory factor analytic approach. Exploratory factor analysis supported a three-factor structure in both samples, which were confirmed by confirmatory factor analysis. Moreover, results supported the internal consistency reliability and convergent validity of the MDDI subscales in both samples. Cumulatively, these findings suggest that the MDDI is an appropriate measure of MD symptoms among cisgender gay men and cisgender lesbian women.