Marijuana use in adolescence is associated with significant adverse outcomes. Romantic relationships are an important context for marijuana use. Prior research suggests a bi-directional relationship between marijuana use and relationship functioning; however, the complex interplay between adolescent relationship dynamics and marijuana use remains unclear. We engaged youth to participate in group model building, a system science approach, to understand from their perspective how social complexities influence the uptake, continuation, and escalation of marijuana use. Two independent groups of clinic and community-recruited youth aged 15-20 participated in a series of four 2-h workshops. Through structured activities, participants generated a causal loop diagram (CLD) representing critical features of the complex and dynamic social system impacting marijuana use for youth in their community. The CLD that emerged represents the mental models of youth and features fourteen feedback loops, including balancing and reinforcing loops, across three domains. These interrelated domains span within-relationship behaviors, factors proximal to marijuana use, and influences on the partner pool, which impact the quality of adolescent romantic relationships and contribute to a high prevalence of marijuana use among youth. Applying a system perspective offers new insights on how stress, and behaviors within relationships in response to stress, feed back to magnify relationship dysfunction and fuel marijuana use. This model provides a new foundation for future research and data collection to better understand and test the identified relationships and feedback loops. Our findings further underscore the importance of educational programs that teach youth about healthy relationship dynamics and stress-coping approaches that do not involve substance use. Understanding how factors function "as a system" provides important information toward illuminating relationship dynamics and designing more impactful and synergistic interventions.