Background: Successful transition from pediatric to adult health care settings supports long-term health management and better overall outcomes in all domains. However, young people with spinal cord injuries (SCIs) continue to report challenges and unmet needs during the transition process. Including end users in health care research and intervention design is paramount as interventions designed in this way better meet their specific needs and are often more innovative. Although studies have reported involving young people with chronic conditions in the development of health care transition (HCT) interventions, few details have been provided as to how this was achieved. Objective: This study outlined the co-design and development of an HCT intervention to support young people with SCIs. It contextualized the co-design process, methods, materials used, and steps implemented from defining the problem to conceiving and designing the solution. This was accomplished by understanding and listening to end users��� needs and recommendations for HCT. Methods: Using participatory methods, this qualitative study reports the co-design of an HCT intervention to support young people with SCIs and parents or caregivers. Two co-design workshops were conducted: one with young people with SCIs and one with parents and caregivers. Categories were defined through a hybrid deductive and inductive qualitative content analysis process that was informed by the Care Transitions Framework and guided the development of the HCT intervention. Following the creation of a prototype intervention, young people with SCIs, parents and caregivers, and key pediatric SCI stakeholders provided feedback on the intervention content and design in focus groups. Similar to the workshops, the focus groups were analyzed using a hybrid deductive and inductive qualitative content analysis process informed by the Care Transitions Framework. The Enhancing the Quality and Transparency of Health Research guidelines for qualitative research (Consolidated Criteria for Reporting Qualitative Research) were applied. Results: Overall, 4 young people and 4 parents or caregivers participated in the co-design workshops. Key recommendations for the HCT intervention were that participants wanted a ���one-stop shop��� for all their transition information needs and an editable portable medical summary to take with them to appointments. On the basis of the analysis of participants��� recommendations from the workshops, it was determined that a website would be an appropriate hosting platform for the interventions. The focus group feedback on the design and content of the prototype website was extremely positive, with minor recommendations for improvement. Conclusions: This is the first study to co-design and develop an HCT intervention in partnership with young people with SCIs and parents and caregivers. Although the study sample was small, it has shown that it is possible to meaningfully engage and empower young people with SCIs and parents and caregivers in the co-design of an HCT intervention.