Despite women’s significant improvement in educational attainment, underrepresentation of women in Science, Technology, Engineering, and Mathematics (STEM) college majors persists in most countries. We address whether one particular institution – single-sex schools – may enhance female – or male – students’ STEM careers. Exploiting the unique setting in Korea where assignment to all-girls, all-boys or coeducational high schools is random, we move beyond associations to assess causal effects of single-sex schools. We use administrative data on national college entrance mathematics examination scores and a longitudinal survey of high school seniors that provide various STEM outcomes (mathematics and science interest and self-efficacy, expectations of a four-year college attendance and a STEM college major during the high school senior year, and actual attendance at a four-year college and choice of a STEM major two years after high school). We find significantly positive effects of all-boys schools consistently across different STEM outcomes, whereas the positive effect of all-girls schools is only found for mathematics scores.