Social media are pervasive in adolescent lives, filled with the potential for youth to experience both risks and opportunities. Social media provide a space for youth to address some of their most salient developmental tasks, such as identity exploration, developing relationships beyond members of their family, and finding places to meaningfully belong in a larger social environment. Social Media TestDrive (SMTD) is an educational intervention designed to develop youth (ages 9–13) knowledge and skills to engage in social media in ways that are safe, positive, prosocial, and fun, in order to maximize benefits while minimizing exposure to risks. SMTD is founded on a research-based definition of social media literacy and delivered through the established pedagogical vehicle of experiential learning theory. This dissertation rigorously evaluates SMTD to assess its effectiveness in developing social media literacy in youth. There are two studies within the dissertation. First, the four-step development (n = 8) and validation (n = 347) of the Youth Social Media Literacy Inventory (YSMLI) yielded an empirically tested, valid, and reliable inventory of objective questions regarding social media literacy knowledge to be used with youth ages 9–13. The YSMLI questions fill a gap in available assessments for both researchers and practitioners. Second, employing questions from the YSMLI results from a cluster randomized controlled trial (n = 435) provides evidence of the effectiveness of SMTD in developing social media literacy knowledge in young learners. This study is the first empirical evaluation of the effectiveness of SMTD. Increasing objective social media literacy knowledge and skills are a foundational first step towards demonstrating positive, safe, and prosocial behaviors in actual social media platforms. According to the SMTD theory of change, completing modules should develop knowledge and skills which will, in turn, affect attitudes, norms, and self-efficacy to ultimately behave in positive, safe, and prosocial ways in social media. The results of this study provide evidence that SMTD is building this foundation of knowledge.