Background: Exercise has consistently yielded short-term, positive effects on health outcomes in people with multiple sclerosis (MS). However, these effects have not been maintained in the long-term. Behaviour change interventions aim to promote long-term positive lifestyle change. This study, namely, "Step it Up" will compare the effect of an exercise plus Social Cognitive Theory (SCT)-based behaviour change intervention with an exercise plus control education intervention on walking mobility among people with MS.Methods/design: People with a diagnosis of MS who walk independently, score of 0-3 on the Patient Determined Disease Steps, who have not experienced an MS relapse or change in their MS medication in the last 12 weeks and who are physically inactive will be randomised to one of two study conditions. The experimental group will undergo a 10-week exercise plus SCT-based behavioural change intervention. The control group will undergo a 10-week exercise plus education intervention to control for contact. Participants will be assessed at weeks 1, 12, 24 and 36. The primary outcome will be walking mobility. Secondary outcomes will include: aerobic capacity, lower extremity muscle strength, participant adherence to the exercise programme, self-report exercise intensity, self-report enjoyment of exercise, exercise self-efficacy, outcome expectations for exercise, goal-setting for exercise, perceived benefits and barriers to exercise, perceptions of social support, physical and psychological impact of MS and fatigue. A qualitative evaluation of Step it Up will be completed among participants post-intervention.Discussion: This randomised controlled trial will examine the effectiveness of an exercise plus SCT-based behaviour change intervention on walking mobility among people with MS. To this end, Step it Up will serve to inform future directions of research and clinical practice with regard to sustainable exercise interventions for people with MS.