This study is a meta-analysis of the effects of out-of-school time (after school, summer camps, enrichment programs, etc.) on the student interest in STEM. This study was guided by the following research questions: (1) How effective is OST as a means to foster student interest in STEM? (2) How does the effectiveness of OST differ by program and study characteristics? A total of 19 independents effect sizes were extracted from 15 studies investigating the effect of out-of-school time (OST) on STEM interest. Included studies were representative of K-12 settings in the United States from 2009-2015. Specifically studies were included if they directly assessed the effects of OST on STEM interest, and provided sufficient data to calculate an effect size. The status of publication was not a constraint on this investigation, thus grey literature was included along with journal articles to provide a more representative sample of studies. The results suggest that out-of-school time has a positive effect on student interest in STEM. Furthermore, the variation in these effects is moderated by program focus, grade level, and the quality of the research design. The effects of out-of-school time on STEM interest are synthesized, and implications for teaching and practice are provided.