Most prostate cancer survivors are confronted with disease-related and treatment-related side effects that impact their quality of life. A tool that combines specific physical activity coaching with the promotion of a healthy lifestyle and self-management guidance might be a successful method to enhance a lifestyle change in these patients. As a prerequisite for useful health technology, it is important to consider a design process centred in the patients. The aim of this study was to investigate the context of the problem and the user needs to support the ideation of a low-fidelity prototype of a tool to promote a healthy lifestyle among early-stage prostate cancer survivors. A user-centred design approach was followed involving a multidisciplinary team. The prototype was developed in 3 phases. In phase 1, the context was studied with 2 systematic reviews of the state of practice and consulting with 3 specialists in Oncology, resulting in a global use case and main requirements. In phase 2, the needs and barriers of the users were studied based on literature research and validated with 3 specialists, resulting in the creation of 3 personas. In phase 3, 2 sessions were held to ideate and prioritize possible app features, based on brainstorming and selection techniques. Using the Ninja Mock and Proto.io software a low-fidelity prototype was developed, resulting in 25 interactive screens. Understanding the user needs and context seems to be essential to highlight key goals, hence facilitating the bridge between ideation of the tool and the intended users tasks and experiences. The conclusion of this first stage of the design process brings valuable details (such as barriers of the users to technology and to physical activity) for future iterations of design of the mobile app.