Abstract: Many veterans experience a decline in their physical activity following their separation from service. Contributory factors include changes in lifestyle, routines and occupational activities, or physical injury, disability, or mental ill health; whether service-related or otherwise. Low physical activity increases risk of serious health consequences, including cardiometabolic disease, which veterans experience at more than two-fold-higher rates than their general population counterparts. Psychological and social factors such as social identities, group norms and social support are important for physical activity uptake and maintenance, but are understudied in veteran populations and their families, especially alongside the incorporation of co-design principles and methods. This protocol paper presents a three-study research program aimed at improved management of cardiometabolic risk for veterans-with a focus on understanding barriers and enablers of physical activity, and culminating in the co-design of a physical activity toolkit. First, a cross-sectional survey will identify risk and protective factors associated with physical activity for veterans and their close family members, including psychosocial factors, such as norms about physical activity within veterans' families. Second, semi-structured interviews with at-risk veterans and family members will further examine perspectives and lived experience of barriers and enablers of physical activity uptake and maintenance. Third, focus group workshops will be undertaken to co-design a toolkit on physical activity that is evidence-based and acceptable for veterans with cardiometabolic risk and their families. Together, the research is expected to generate a new understanding of how best to support veterans and their families with uptake and maintenance of physical activity, particularly those with cardiometabolic risk indicators.