This article examines a biosocial model of the impact of puberty on indigenous girls' early-onset substance use by considering the potential mediating role of peer context (i.e., mixed-sex peer groups and substance use prototypes) on the puberty and substance use relationship. Data include responses from 360 girls of a common indigenous cultural group residing on reservations/reserves in the upper Midwest and Canada. Results of structural equation modeling revealed that the statistically significant relationship between girls' pubertal development and early-onset substance use was mediated by both mixed-sex/romantic peer groups and favorable social definitions of substance use. Implications for substance use prevention work include addressing the multiple and overlapping effects of peer influence from culturally relevant perspectives. (Contains 2 figures, 2 tables, and 1 note.)