Background: Seventy percent of lifetime cases of mental illness emerge before the age of 24 years, but many youth are unable to access the support and services they require in a timely and appropriate way. With most youth using the internet, electronic health (eHealth) interventions are promising tools for reaching this population. Through participatory design research (PDR) engagement methods, Thought Spot, a Web- and mobile-based platform, was redeveloped to facilitate access to mental health services by transition-aged youth (aged 16-29 years) in postsecondary settings. Objective: The aim of this study was to describe the process of engaging with postsecondary students through the PDR approaches, with the ultimate goal of optimizing the Thought Spot platform. Methods: Consistent with the PDR approaches, five student-led workshops, attended by 41 individuals, were facilitated to obtain feedback regarding the platform’s usability and functionality and its potential value in a postsecondary setting. Various creative engagement activities were delivered to gather experiences and opinions, including semistructured focus groups, questionnaires, personas, journey mapping, and a world café. Innovative technological features and refinements were also brainstormed during the workshops. Results: By using PDR methods of engagement, participants knew that their ideas and recommendations would be applied. There was also an overall sense of respect and care integrated into each group, which facilitated an exchange of ideas and suggestions. Conclusions: The process of engaging with students to redesign the Thought Spot platform through PDR has been effective. Findings from these workshops will significantly inform new technological features within the app to enable positive help-seeking behaviors among students. These behaviors will be further explored in the second phase that involves a randomized controlled trial. [J Med Internet Res 2018;20(3):e79]