Numerous studies have demonstrated the negative influence of sexual debut during adolescence on mental health outcomes. This article contributes to this literature by investigating whether sexual debut has negative effects on mental health among South Korean adolescents and whether the timing of adolescent sexual debut matters. Drawing on longitudinal data from a nationally representative survey, we first predicted mental health outcomes at one year after high school graduation using first sexual intercourse that had occurred before the outcomes were measured. In a second statistical model, adolescent sexual debut was defined as first coitus that had occurred before high school graduation. Sexual debut was associated with an increase in problematic aggressive behaviors for both genders. In contrast, only girls experienced a rise in depressive symptoms after becoming sexually active. For girls, having sex before high school graduation was correlated with worse mental health outcomes to the extent that sexual debut even enhanced the risk of suicidal ideation. We concluded that the negative effects of sexual activity among South Korean adolescents are attributable mainly to the sexually conservative atmosphere and gendered sexuality in that country.