This study had three goals: (a) assessing the predictive association of externalizing and internalizing behaviors during childhood with sexual onset during early adolescence; (b) examining the interactive link of externalizing and internalizing behaviors with early sexual onset; and (c) investigating the moderating effect of gender in this context. A sample of 343 French-Canadian participants (140 girls) was followed annually from kindergarten until age 15. Externalizing and internalizing behaviors during childhood were reported by teachers, whereas early sexual onset was self-reported from age 13 to 15. Controlling for sociofamily risk, pubertal status, social preference and sexual abuse, results showed a significant interactive link between externalizing and internalizing behaviors with early sexual onset, albeit only among boys. Specifically, boys high on externalizing and low on internalizing problems in childhood were at increased risk of early sexual onset in early adolescence, whereas externalizing boys with high levels of internalizing problems were not. Externalizing and internalizing problems did not predict early sexual onset for girls. The developmental and clinical implications of these findings are discussed.