Overweight and obesity are globally challenging health problems and major risk factors for a number of chronic diseases, including diabetes, cardiovascular diseases and cancer. Sufficient physical activity (PA) is an important protective factor in the development of overweight and obesity. Adolescents with intellectual disability (ID) report lower levels of PA and higher rates of overweight and obesity in comparison to adolescents without ID. There is a dire need to find ways to promote PA in adolescents with ID for their present and future health. However, while PA promotion in adolescents is a well-developed science, there is only a handful of studies dedicated to adolescents with ID. Consequently developing and designing PA promotion interventions specifically tailored to the needs of youth with ID is of the utmost importance. Currently, however, there is a real knowledge gap in terms of fundamental research on crucial intervention mechanisms to promote PA in adolescents with ID, as well as specific behaviour change theories adapted to this population. The aim of the current research project is to co-create and evaluate a theory-based mHealth intervention, which focuses on increasing PA for adolescents with ID. By doing so, health and research inequalities will be tackled. A mixed-method approach will be applied using two studies with an ABAB reversal design, log data analysis, questionnaires and process evaluation through semi-structured interviews.