Unstructured out-of-school time (OST) science activities, such as reading a science book, watching a science television show, or researching on the internet about science, constitute a self-sustaining way for adolescents to engage in science, technology, engineering, and mathematics (STEM). Although it is suggested that long-term engagement in such activities could have a broad influence on several STEM-related constructs, so far little is known about the impact of unstructured OST science engagement. The current study therefore investigated the effects of unstructured OST science engagement on the development of vocational interests, occupational aspirations, competencies, school achievement, and ability self-concepts. For this purpose, we used a large longitudinal subsample (N = 2,655) from the National Educational Panel Study (NEPS; Blossfeld & Roßbach, 2019) where students in Germany were assessed in Grades 9, 11, and 12. Following the recommendations of VanderWeele et al. (2020), we used an outcome-wide longitudinal design for causal inference: Outcome-wide effects of unstructured OST science activities were estimated while controlling for a set of joint confounders and pretest measures. Our findings show that unstructured OST science activities influence investigative vocational interests, but do not influence occupational aspirations, competencies, school grades, and ability self-concepts. The results suggest that adolescents with similar initial interest trait levels who engage in unstructured OST science activities develop a stronger interest toward STEM, compared to adolescents who do not engage in such activities.