We study how educational opportunities change adolescents' gender attitudes in Tanzania, using an experiential education program focused on STEM subjects. After the intervention, girls' gender attitudes became more progressive by 0.29 standard deviations, but boys' gender attitudes did not change. Perceived improvement in the labor market opportunities appears to be an important channel to explain the result. The intervention also increased girls' weekly study hours and boosted their interests in STEM-related subjects and occupations. Our results show that providing STEM-related educational opportunities to girls in developing countries can be an effective way of improving their gender attitudes.