ObjectiveTo develop a mobile health app to assess individual-specific meaningful societal participation in real time and to evaluate its usability. DesignDevelopment and usability study. SettingGeneral community. ParticipantsPersons with multiple sclerosis (PwMS) utilized the app for 7 consecutive days. In total, 72 PwMS and smartphone owners were included in the analysis (N=72). InterventionsUsing location tracking, the newly developed Whereabouts app generates an individual-specific timeline of societal participation activities each day, consisting of location and transportation intervals. Subsequently, this timeline is real time enriched by the user with self-reported ratings of meaningfulness and perceived strain of these societal participation activities. The app is based on the International Classification of Functioning, Disability and Health and was developed in an iterative process. Main Outcome Measure(s)Usability of the newly developed Whereabouts app was evaluated by analyzing the effectiveness, efficiency, and user satisfaction. ResultsRegarding effectiveness, the app correctly assessed the type, frequency, and duration of different societal participation activities for 96.1% of the participation activities. The self-reported ratings of societal participation varied for meaningfulness (range, 5-8), perceived strain (range, 2-6), and meaningfulness of the perceived strain (range, 5-8). The latter 2 were highly correlated (r=0.857). With regards to efficiency, 3.9% of the generated participation activities had to be excluded due to inaccuracy or incompleteness. Relating to user satisfaction, 57.3% of PwMS reported that they were satisfied with the usability of the app and 59.7% stated that it gave a realistic overview of their daily activities. However, 54.4% PwMS missed the possibility to specify activities at home, to add activities in more detail, and to correct mistakes. ConclusionsThe Whereabouts app demonstrates usability in assessing real-time, individual-specific meaningful societal participation. Improvements are recommended, such as the possibility to specify participation activities and to generate a graphic overview.