We examine individual-level determinants of interest in STEM and analyze if a digital web application for elementary-school children can increase children's STEM interest with a specific focus on narrowing the gender gap in STEM. Coupling a randomized-controlled trial with experimental lab and survey data, we can analyze the effect of the digital intervention and shed light on the mechanisms. We confirm the hypothesis that girls demonstrate a lower overall interest in STEM than boys. Moreover, girls are less competitive and exhibit less pronounced math confidence than boys at the baseline. Our treatment increases girls' interest in STEM and decreases the gender gap via an increase in STEM confidence and competitiveness. Our findings suggest that an easy-to-implement digital intervention has the potential to foster gender equality for young children and can contribute to a reduction of gender inequalities in the labor market such as occupational sorting and the gender wage gap later in life.