Objective: This study aimed to design a persuasive game, using objective adherence data, to motivate people with asthma to adhere to their medication regimen. Methods: A participatory user-centered design approach was employed, involving end-users and other stakeholders throughout the study. The approach consisted of four phases. Semi-structured interviews and a survey were conducted to understand user needs and reasons for poor adherence (Phase 1: define). Key themes were identified, leading to the formulation of behavior change strategies and design and game requirements. Several design directions were ideated, resulting in a concept for a serious game (Phase 2: ideate). Two rounds of user-tests were performed to evaluate a prototype of the serious game in terms of usability, perceived impact on medication adherence and motivation (Phase 3: prototype and Phase 4: evaluate). Results: Findings from semi-structured interviews (n = 6) and the online survey (n = 20) revealed that people’s non-adherence was often attributed to the perception of asthma as an episodic condition, the delayed experienced effect of maintenance inhalers, and lack of knowledge regarding difference of effect between maintenance and reliever inhalers. The study used behavior change strategies to translate these insights into design requirements for the development of the narrative-based persuasive game Ademgenoot. This six-week challenge-based game combines various behavior change strategies, including personal goal setting and continuous visual feedback, as well as persuasive game design elements, such as a narrative and rewards, with the aim of enhancing motivation to adhere to their medication regimen. User-testing (n = 8; round 1 and 2) showed that Ademgenoot is feasible in clinical practice and has the potential to support people with mild asthma in adherence to their maintenance medication. Discussion: Future efforts should be directed towards a larger evaluation to assess the impact on motivation and inhaler use behaviour. Plain Language Summary: The goal of this study was to create a serious game that encourages people with asthma to take their medication regularly. During the study, we worked closely with individuals who have asthma and other stakeholders throughout the study. We conducted interviews and surveys to understand why people have difficulties using their maintenance inhaler as prescribed by their doctor. Based on the feedback we received, we developed a serious game called “Ademgenoot”. The game uses information on inhaler use automatically collected with a device attached to the inhaler. The game includes features like personal goals and visual feedback on inhaler use to motivate users to take their medication consistently. We tested a prototype of the game with users to see if it was easy to use and if it motivated them to use their maintenance inhaler. The results showed that Ademgenoot is a viable option for helping individuals with mild asthma stay on track with their medication.