BACKGROUND: Daytime urinary incontinence (UI) is common in childhood and often persists into adolescence. UI in adolescence is associated with a range of adverse outcomes, including depressive symptoms, peer victimization, poor self-image, and problems with peer relationships. The first-line conservative treatment for UI is bladder training (standard urotherapy) that aims to establish a regular fluid intake and a timed schedule for toilet visits. The success of bladder training is strongly dependent on good concordance, which can be challenging for young people. OBJECTIVE: This paper aims to describe the development of a smartphone app (URApp) that aims to improve concordance with bladder training in young people aged 11 to 19 years. METHODS: URApp was designed by using participatory co-design methods and was guided by the person-based approach to intervention design. The core app functions were based on clinical guidance and included setting a daily drinking goal that records fluid intake and toilet visits, setting reminders to drink fluids and go to the toilet, and recording progress toward drinking goals. The development of URApp comprised the following four stages: a review of current smartphone apps for UI, participatory co-design workshops with young people with UI for gathering user requirements and developing wireframes, the development of a URApp prototype, and the user testing of the prototype through qualitative interviews with 23 young people with UI or urgency aged 10 to 19 years and 8 clinicians. The app functions and additional functionalities for supporting concordance and behavior change were iteratively optimized throughout the app development process. RESULTS: Young people who tested URApp judged it to be a helpful way of supporting their concordance with a timed schedule for toilet visits and drinking. They reported high levels of acceptability and engagement. Preliminary findings indicated that some young people experienced improvements in their bladder symptoms, including a reduction in UI. Clinicians reported that URApp was clinically appropriate and aligned with the best practice guidelines for bladder training. URApp was deemed age appropriate, with all clinicians reporting that they would use it within their own clinics. Clinicians felt URApp would be of particular benefit to patients whose symptoms were not improving or those who were not engaging with their treatment plans. CONCLUSIONS: The next stage is to evaluate URApp in a range of settings, including pediatric continence clinics, primary care, and schools. This research is needed to test whether URApp is an effective (and cost-effective) solution for improving concordance with bladder training, reducing bladder symptoms, and improving the quality of life.