The purpose of this study was to examine substance use among a racially and ethnically diverse group of HIV-positive men who have sex with men (MSM) living in six U.S. cities, model associations between drug use and serodiscordant unprotected anal intercourse (SDUAI), and characterize users of the substances strongly associated with risky sexual behavior. Baseline questionnaire data from 675 participants of the Positive Connections intervention trial were analyzed. Overall, substance use was common; however, the highest percentage of stimulant (30%), methamphetamine (27%), and popper (i.e., amyl nitrite) (46%) use was reported among white MSM and crack/cocaine (38%) use was highest among African American MSM. Popper use versus non-use (odds ratio = 2.46; 95% confidence interval = 1.55-3.94) and condom self-efficacy (1 standard deviation (sd) increase on scale; odds ratio = .58; 95% confidence interval = .46-.73) were significantly associated with SDUAI after adjusting for key demographic and psychosocial factors. These results highlight the importance of addressing drug use in the context of sex for possible HIV transmission risk.