Background: Little is known about the influence of contextual characteristics on comorbid substance use and serious mental illness (SMI). Aims: To explore the role of poverty on comorbid SMI and cannabis use. Methods: We used data from the 2015 National Survey on Drug Use and Health, considering those in poverty, with income under 100% of the US poverty threshold. Results: People in poverty were more likely to suffer from concurrent SMI and cannabis use (3.07%, 95% confidence interval (CI):1.84%; 5.07%), even controlling for gender, age, tobacco and alcohol use (odds ratio (OR) = 2.77, 95% CI: 1.27; 6.03, p = .010). Conclusion: The magnitude of the association between SMI and cannabis use is influenced by poverty status. More research on potential mediators like income inequality and impoverished social capital is needed.