Aims To test two indirect pathways through which sexualminority adolescents (SMAs)may be at risk for heavy episodic drinking (HED) including a socialization pathway via substance-using peer affiliations and social marginalization pathway via sexual minority-specific victimization and subsequent substance-using peer affiliations. Design Analysis of the first three waves (6 months apart) of a longitudinal adolescent health risk study (2011--14). Participants were referred by medical providers or a screening system in providers' waiting rooms. Setting Two large urban adolescent health clinics in Pennsylvania and Ohio, USA. Participants A total of 290 adolescents (ages 14--19 years, mean: 17.08) who were 71.0% female, 33.4% non-Hispanic white and 34.5% SMAs. Measurements Self-reported sexual minority status (wave 1) and affiliation with substance-using peers (waves 1 and 2), and latent sexual-minority specific victimization (waves 1 and 2) and HED (waves 1 and 3) variables. Findings Using mediation analyses in a structural equation modeling framework, there was a significant indirect effect of sexual minority status (wave 1) on HED (wave 3) via affiliation with substance-using peers [wave 2; indirect effect = 0.03, 95% confidence interval (CI) = 0.01, 0.07], after accounting for the indirect effect of sexual-orientation related victimization (wave 2; indirect effect = 0.10, 95% CI = 0.02--0.19). The social marginalization pathway was not supported, as victimization (wave 1) was not associated with affiliation with substance-using peers (wave 2; β = -0.04, P = 0.66). Sex differences in the indirect effects were not detected (Ps>0.10). Conclusions Sexual minority adolescents in the United States appear to exhibit increased heavy episodic drinking via an indirect socialization pathway, including affiliations with substance-using peers and a concurrent indirect pathway involving sexual minority-related victimization. The pathways appear to operate similarly for boys and girls.