Background: Adolescence is a sensitive period for the onset of mental health disorders. Effective, easy-to-disseminate, scalable prevention and early interventions are urgently needed. Affective control has been proposed as a potential target mechanism. Training affective control has been shown to reduce mental health symptoms and improve emotion regulation. However, uptake and adherence to such training by adolescents has been low. Thus, the current study aims to receive end user (i.e., adolescents) feedback on a prototype of a novel app-based gamified affective control training program, the Social Brain Train. Methods: The proposed study aims to recruit participants aged 13-16 years old (N = 20) to provide user feedback on the Social Brain Train app. The first group of participants (n = 5) will complete an online questionnaire assessing demographics, symptoms of depression and anxiety, social rejection sensitivity and attitudes toward the malleability of cognition and mental health. They will complete two tasks assessing cognitive capacity and interpretation bias. Participants will be then be invited to an online group workshop, where they will be introduced to the app. They will train on the app for three days, and following app usage, participants will complete the aforementioned measures again, as well as provide ratings on app content, and complete a semi-structured interview to obtain in-depth user feedback, which will be used to inform modifications to the app. Following these modifications, a second group of participants (n = 15) will follow the same procedure, except they will train on the app for 14 days. Feedback from both groups of participants will be used to inform the final design. Conclusions: By including young people in the design of the Social Brain Train app, the proposed study will help us to develop a novel mental health intervention that young people find engaging, acceptable, and easy-to-use