Muscle dysmorphia (MD) is characterized by a pervasive belief or fear of insufficient muscularity and an elevated drive for muscularity, representing the pathological and extreme pursuit of muscularity. Psychometric properties of one of the most widely used measures of MD symptoms—the Muscle Dysmorphic Disorder Inventory (MDDI)—have yet to be evaluated in transgender men despite emerging evidence suggesting differential risk for MD symptoms in this population. In this study, we assessed the psychometric properties of the MDDI in a sample of 330 transgender men ages 18–67 years who participated in a large-scale national longitudinal cohort study of sexual and gender minority adults in the U.S. Using a two-step, split-sample approach, an initial exploratory factor analysis supported a three-factor structure and a subsequent confirmatory factor analysis of a re-specified three-factor model demonstrated good overall fit (χ2/df = 1.84, CFI =0.94, TLI =0.92, RMSEA =0.07 [90% CI =0.05,.09], SRMR =0.08). Moreover, results supported the internal consistency and convergent validity of the MDDI subscales in transgender men. Findings inform the use of the MDDI among transgender men and provide a foundation to support further work on the MDDI and MD symptoms among gender minority populations.