Objective: This study examined whether the association between social support and condom self-efficacy would be moderated by (a) internalized heterosexism among and (b) enacted heterosexism experienced by young Black men who have sex with men (YBMSM), who contend with high HIV incidence, heterosexism, and low uptake of preexposure prophylaxis. Method: Participants were 1,210 YBMSM (ages 18–29) who completed measures of social support, internalized and enacted heterosexism, and condom self-efficacy in 2 large cities in the southern United States as part of a community-level HIV-prevention study. Results: A significant 3-way interaction between social support and both hypothesized moderators, internalized and enacted heterosexism, showed that social support was positively associated with condom self-efficacy when both internalized and enacted heterosexism were high (1 SD above the mean; b = .177, 95% confidence interval [CI: .088, .266]). However, social support was not associated with condom self-efficacy when scores were low (1 SD below the mean) on both internalized and enacted heterosexism (b = .024, 95% CI [−.054, .101]), low on internalized and high on enacted heterosexism (b = .058, 95% CI [−.061, .117]), or high on internalized and low on enacted heterosexism (b = .039, 95% CI [−.083, .161]). Conclusions: YBMSM who are high in both internalized and enacted heterosexism may see greater benefits from social support on condom self-efficacy than would YBMSM who grapple with less heterosexism. In addition to promoting social support, interventions should aim to assess and reduce multiple forms of stigma.