Purpose: Sexual minority individuals have heightened risk for substance use; however, previous studies have not assessed severity of alcohol use disorders (AUDs), tobacco use disorders (TUDs), and drug use disorders (DUDs) among lesbian/gay and bisexual individuals and those “not sure” of their sexual identity compared with heterosexual individuals. This study examined how three dimensions of sexual orientation (identity, attraction, and behavior) relate to severity of AUD, TUD, and DUD.Methods: This study used cross-sectional national data (N = 36,309) from the National Epidemiologic Survey on Alcohol and Related Conditions-III, and well-validated alcohol, tobacco, and drug measures that align with Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition criteria. Data were collected through in-person interviews in 2012-2013.Results: Sexual minority respondents, based on sexual identity, had higher odds of severe AUD or TUD than heterosexual respondents. Opposite- and same-sex behavior was a predictor of severe AUD (adjusted odds ratio [AOR] = 2.44; 95% confidence interval [CI] = 1.24-4.79) and TUD (AOR = 2.16; 95% CI = 1.19-3.93), but not DUD. Those “not sure” of their sexual identity had higher odds of severe AUD, TUD, and DUD: AUD (AOR = 5.05; 95% CI = 2.78-9.16), TUD (AOR = 4.18; 95% CI = 2.29-7.64), and DUD (AOR = 4.40; 95% CI = 1.72-11.2), than heterosexual respondents. There were few significant differences between “not sure” and bisexual respondents.Conclusions: These findings provide strong evidence that bisexual and “not sure” U.S. adults are more likely to have a severe AUD and TUD. They also demonstrate the importance of treatment strategies that address sexual minority-specific risks, particularly for bisexual individuals and those “not sure” of their sexual identity.