Establishing the discriminative factors for unprotected sex among adolescents is essential for early identification of at-risk teens and for the prevention of unplanned pregnancy and sexually transmitted diseases. The aim of this study was to examine the discriminative effects of demographic, individual, family, peers, and school life factors on unprotected sex in a large-scale, representative adolescent population in Southern Taiwan. A total of 9,736 adolescent students were recruited into this study and completed the questionnaires. The multidimensional discriminative factors for unprotected sex were examined using chi2 automatic interaction detection analysis and logistic regression models. The results of the chi2 automatic interaction detection analysis revealed that having friends, using illicit drugs, being of an older age, suspension from school, and low family monitoring had discriminative effects on unprotected sex in adolescents. The logistic regression analysis further confirmed the discriminative effect of these factors. Because of the adverse effects of unprotected sex in adolescents, we suggest that parents and health professionals should pay attention to adolescents with the discriminative factors for unprotected sex identified in this study.