Childhood cancer survivors (CCSs) treated with cardiotoxic cancer treatments are at increased risk of developing cardiometabolic complications. This risk is further exacerbated by poor health behaviours. In particular, CCSs are less active than non-cancer comparators. Existing interventions aiming to improve physical activity (PA) levels in CCSs are methodologically weak. The aim of this study is to rigorously and systematically develop an evidence-based and theoretically-informed intervention to promote, support, improve and sustain PA levels in CCSs, with the long-term goal of reducing CCSs’ cardiovascular morbidity and mortality.