<strong>Background:</strong> The transition from child to adult mental health services is a vulnerable period marked by service disengagement, care gaps, and worsening mental health outcomes. Although planned, developmentally appropriate transition processes can improve functioning, youths report insufficient preparation, limited continuity of care, and unmet expectations for support. Existing transition supports remain underevaluated and require further adaptation for mental health contexts. Youth consistently report needing clearer information, concrete support, and sustained connection. Digital tools, particularly SMS text messaging, which is widely used, accessible, and acceptable to youth, offer a promising way to deliver timely transition supports. Yet most digital mental health tools are developed without meaningful youth involvement, highlighting the need for participatory approaches to ensure relevance, usability, and uptake. <strong>Objective:</strong> This study aimed to co-design and refine prototypes for a transition-focused SMS text messaging intervention by engaging youth with lived experience in a participatory co-design activity (design jam) to identify priority content, key functionality, and implementation enablers to support the transition from child to adult mental health services. <strong>Methods:</strong> We conducted a 3-hour mixed methods, participatory design jam to co-design transition-focused SMS text messaging prototypes, recruiting youth aged 16-26 years in Canada who had recently transitioned to, or were approaching transitioning to, adult mental health services. Data sources included workshop artifacts, observational field notes, and audio recordings from structured activities involving evidence review, brainstorming, rapid prototyping, brief team pitches, and evaluation. Rapid qualitative analysis, integrating open coding, content analysis of visual prototypes, and the rapid identification of themes from audio recordings, was used to identify priority content, key functionality, and implementation enablers. Findings were refined through a member-checking debrief with youth participants. <strong>Results:</strong> Seven youths aged 19-24 years participated in the design jam. Across two teams, participants generated 54 content ideas and 50 feature ideas. Two distinct prototypes were developed: one emphasizing long-term affirmation, self-advocacy, self-care, and profile-based customization, and the other prioritizing shorter-term informational support, navigation resources, and flexible message frequency. Youth across both groups highlighted the importance of interactive and visually engaging elements. Analysis revealed 3 thematic tensions shaping youth design preferences: balancing autonomy with ongoing support (roaming/reconnecting), balancing personalization with the need for simplicity (customization/convention), and balancing knowledge delivery with motivation for action (learning/living). Participants rated the design jam positively. <strong>Conclusions:</strong> Youth meaningfully contributed to co-designing an SMS text messaging intervention to support transition from child to adult mental health services, generating concrete content, functionality, and implementation priorities. Their prototypes highlighted the need to balance autonomy with support, personalization with simplicity, and information with motivational guidance. These findings demonstrate the value of participatory co-design in developing youth-centered digital transition supports and underscore the importance of evaluating such prototypes in real-world settings to determine feasibility and impact.
