Regular exercise is beneficial for youth’s physical and mental health. Still, the majority is insufficiently active. Physical education teachers and more specifically their motivating style play a crucial role in motivating young people to be active. Hence the need for effective interventions targeting teachers’ motivating style. Current Self-Determination-Theory-based interventions are mostly workshop-like one-size-fits-all programs. In this project, we aim to (1) develop an online tailored tool to optimise secondary school teachers’ motivating style (Phase 1) and (2) evaluate its effects on student- and teacher-outcomes (Phase 2). In Phase 1, we will conduct cross-sectional (Study 1) and design-based (Study 2) research. In Study 1, tailoring principles will be developed by comparing teachers’ self-reports, students' perceptions (questionnaire), and observations (class video-recordings) of teachers’ motivating style. In Study 2, teachers' desires concerning an online tool will be identified through interviews and questionnaires. In Phase 2, we will evaluate the tool in a randomised-controlled-trial (Study 3). Effects on student- (need-satisfaction/frustration, motivation for physical education, and physical activity) and teacher- (motivating style and well-being) outcomes will be examined. Groundbreaking is the online tailoring based on multi-informant measures and the potential impact on health promotion and educational practice.