Marijuana use in the United States (U.S.) has diversified alongside expanding legalization, yet little is known about the psychosocial factors that distinguish medical from recreational use. This study examined whether psychological distress mediates the association between perceived social isolation and marijuana use type among U.S. adults. We analyzed cross-sectional, nationally representative data from the 2024 Health Information National Trends Survey (HINTS, cycle 7). Marijuana use was categorized as medical (including medical and both medical/recreational) versus recreational. Perceived social isolation was measured using the Patient-Reported Outcomes Measurement Information System (PROMIS) Social Isolation t-score, and psychological distress was assessed with the Personal Health Questionnaire (PHQ)-4. Sur-vey-weighted descriptive analyses and a structural equation mediation model accounting for the complex sampling design were conducted. Medical marijuana users reported significantly higher levels of psychological distress and perceived social isolation than recreational users. Greater social isolation was strongly associated with higher psychological distress, and higher distress was associated with a greater likelihood of medical (vs. recreational) marijuana use. The indirect effect of social isolation on marijuana use type through psychological distress was statistically significant, while the direct effect of social isolation was not significant after accounting for distress. Overall, greater perceived social isolation predicted medical marijuana use primarily through elevated psychological distress. These findings suggest that medical marijuana use among U.S. adults may reflect coping with psychological distress linked to social disconnection, under-scoring the importance of integrating mental health and social context in-to clinical and public health approaches to cannabis use.