BackgroundGastric cancer is one of the most common gastrointestinal cancers. Patients with gastric cancer experience disabilities and complications that lead to reduced quality of life. Empowering these patients by providing them with information and self-management skills can help reduce side effects and improve their quality of life.ObjectiveThe aim of this study was to identify the user requirements for developing a mobile-based self-management app to support patients with gastric cancer.MethodsData were analyzed using descriptive statistics and frequency distribution reports using IBM SPSS Statistics software.ResultsAll of the data elements and functional requirements except “data recording times” and “weight changes in graphs” were identified as essential by clinical experts and patients. Among the functional requirements required in a gastric cancer self-management app, the capabilities related to informing, announcing warnings, and reminders are included. In the demographic data section, most patients (14/26, 53%) did not comment on the importance of recording data such as name, surname, and place of residence, and the demographic data section was met with less agreement from patients than clinicians.ConclusionsApplying the requirements mentioned in this study can improve the self-management of patients with gastric cancer. Such apps can play an important role in empowering patients and improving their quality of life. However, the apps need to be designed and implemented to see how they can meet users’ requirements.