In recent years, many mobile health (mHealth) apps have been developed to support diagnostics and treatment, among other purposes. It is likely that involving patients closely in the development process will lead to more relevant apps. In theory, the Agile style of software development can make this possible. However, whether this is feasible in mental healthcare practice has never been investigated.<br/> AIM: To investigate whether it is possible in practice to develop an mHealth app together with young people in a highly specialized mental healthcare context, by using Agile.<br/> METHOD: A proof-of-concept study that seeks to develop an mHealth app by implementing Agile together with clinically admitted, young psychiatric patients. Patients would directly influence the development goals and priorities.<br/> RESULTS: In the period from May to July 2019 the app 'Constant Circles' has been developed using Agile together with patients. The main focus of this app is social support. The patients supplied 18 concrete user stories and also provided feedback with general principles for developing an mHealth app.<br/> CONCLUSION: This study has demonstrated that it is possible to closely involve patients in highly specialized mental health care in the development process of an mHealth app by using Agile.