Insomnia symptoms have been linked to problematic marijuana use among young adults, but the mechanism underlying this association and whether sex differences exist, remains unclear. Using cross-sectional data, this study examined negative mood as a mediator of the association between insomnia and marijuana problems among male and female college students. Undergraduate students (n = 267; 61% female) reporting marijuana use in the past month completed an online survey assessing insomnia symptoms, negative mood and marijuana problems. Controlling for relevant covariates, negative mood was examined as a mediator of the association between insomnia and marijuana problems using bootstrapped significance tests for indirect effects (n-boot = 1,000). Results indicated that higher levels of insomnia were associated with greater levels of negative mood (regardless of sex), which in turn were associated with greater marijuana-related problems. In conclusion, insomnia symptoms are associated with more negative mood among college students who use marijuana, and this effect on negative mood accounts for a large part of the association of insomnia symptoms with marijuana-related problems. Research is needed to determine if these associations are maintained prospectively.© 2020 European Sleep Research Society.