Background: Assistive technology (AT) has the potential to support and enhance self-management of people living with dementia. However, a range of special and heterogeneous needs must be considered when designing and deploying AT for people with dementia, and consequently the involvement of end-users throughout the design process is essential to provide usable and effective AT solutions. Objective: The ReACT study was conducted to investigate how a tailor-made app, the ReACT app, can be designed and deployed to meet the needs of people with dementia in relation to self-management. Methods: This paper presents 4 steps of an iterative user-involving app design process. In the first step, a pilot study was conducted to explore the potential benefits and challenges of using existing off-the-shelf apps to support self-management when living with early-stage dementias. In the second step, focus group interviews provided in-depth understanding of the perspectives and needs of potential end-users of the app. The third step was a product benchmarking process, which served to further qualify the design process. Finally, results from these first 3 steps were included in the fourth step where the ReACT app was designed through an iterative codesign process. In total, 28 people with dementia, 17 family caregivers, and 10 professional caregivers were involved through these 4 iterative steps. Results: The functionalities and the design of the ReACT app directly reflect the perspectives and needs of end-users in relation to self-management. Support of memory and structure in daily living were identified as main needs, and the ReACT app was designed as a holistic and adaptable solution with a tailor-made calendar as a key feature. Conclusion: Based on this extensive iterative user-involving design process, the ReACT app has great potential to support and enhance self-management of people living with dementia. Further studies are needed to test and validate the usability and impact of the app, and methods for deployment and adoption of AT for people with dementia also need to be considered.