Using a national longitudinal data from South Korea, this study investigates the effects of single-sex schooling on three non-cognitive outcomes: depressive symptoms, self-esteem, and school aspirations. Analysis using a differences-in-differences approach reveals that single-sex school attendance improves non-cognitive outcomes, especially for girls, but the results are mixed for boys. These findings are robust across varying definitions of treatment and control groups utilizing single-gender and mixed-gender classes in coeducational schools. While coeducational schooling is attracting greater attention in latedeveloped countries such as South Korea, this study implies that it might have unintended adverse effects on students’ non-cognitive development.