Little research has examined mechanisms driving cannabis use disparities among sexual minority (SM; versus heterosexual) young adults (YAs). Participants were US YA women (N = 1345; 24.8% bisexual, 6.1% lesbian) and men (N = 998: 8.9% bisexual, 13.8% gay). Bivariate analyses examined associations between sexual orientation (heterosexual, gay/lesbian, bisexual) and cannabis use outcomes (past 30-day [current] use, use frequency, cannabis-tobacco dual use, use intentions), perceived risks (harm, addictiveness) and social norms (social acceptability, peer use). Regressions examined sexual orientation, perceived risks and social norms as correlates of cannabis outcomes. Bisexual and lesbian (versus heterosexual) women reported lower cannabis-related perceived risks, greater social norms and higher odds of current use. Bisexual (versus heterosexual) women reported more days of use, higher odds of cannabis-tobacco dual use and greater use intentions. Gay (versus heterosexual) men reported lower perceived addictiveness, greater social norms and higher odds of current use. Lower perceived risks and greater social norms predicted greater use outcomes for women and men. Bisexual women displayed higher odds of several cannabis use outcomes, whereas lesbian women and gay men displayed higher odds of current cannabis use only. Cannabis-related perceptions and social norms may be important targets for public health messaging for SMYAs.