Voting is a critical component of representative democracy but yields consistently low engagement from young Americans. Online communication interventions have the potential to catalyze action, but evidence regarding best practices for digital, youth-focused get-out-the-vote (GOTV) messaging is sparse. Guided by the Theory of Planned Behavior and Reasoned Action approaches, we integrated formative and participatory research approaches to fill this gap. In a partnership between communication researchers and a college student-led GOTV organization, we conducted formative research integrating experiential knowledge from youth partners with scientific evidence to generate a set of voting beliefs. We used the standard Hornik & Woolf method to select promising beliefs to target in messaging but also conducted post-hoc analyses demonstrating the potential of network approaches that take into account relationships among beliefs to identify influential beliefs. We then co-designed belief-informed GOTV messages that could be disseminated via social media and tested message effects in both local and national samples of college students. We found that the messages co-designed with youth to target promising voting beliefs were more motivating, persuasive, and self-relevant than existing GOTV social media messages from a national youth-focused campaign. They were also more motivating and persuasive than prior messages from our student-led GOTV partner organization—which were not designed using formative research—suggesting that the combination of formative and participatory research practices increases message effectiveness. Overall, these studies highlight the importance of engaging youth in voting research and illustrate the benefits of integrating theory-based formative research, network approaches, and human-centered participatory practices to develop messaging campaigns.