© Author(s) (or their employer(s)) 2019. Introduction People with multiple sclerosis (MS) are less physically active, and more sedentary than their peers despite evidence that activity helps to manage MS-related symptoms. Traditional approaches to increasing physical activity, such as exercise programmes, can be challenging for people with MS, especially those with walking disability. Focusing on decreasing prolonged sitting, and increasing light-intensity activities may be more feasible and result in more sustainable behaviour change in persons with MS. This paper describes the rationale and development of a sedentary behaviour intervention targeting persons with MS. Methods and analysis The feasibility and preliminary efficacy of a sedentary behaviour intervention will be tested using a prepost intervention design in 40 adults with MS. The 22-week programme includes a 15-week intervention and a 7-week follow-up. The intervention itself is divided into two stages: Sit-Less and Move-More. The Sit-Less stage is designed to encourage participants to break up prolonged sitting bouts, while the Move-More stage promotes increasing steps per day, in addition to interrupting sitting. The intervention is delivered through individual coaching sessions between an interventionist and a participant, and an accompanying newsletter based on social cognitive theory. A Fitbit is used to monitor activity throughout the programme. Process, resource and management metrics will be recorded (eg, retention, time required for communication during the trial). Sedentary and physical activities and MS-related symptoms are measured before and after the intervention and again during follow-up. Experiences with the programme are explored through an online survey and one-on-one interviews. Ethics and dissemination The Health Research Ethics Board at the University of Alberta granted permission to conduct this study. Results will be disseminated in scientific journals and conferences, and the MS Society of Alberta. Physical therapists and kinesiologists are important stakeholders and will be targeted during dissemination. Trial registration number NCT03136744.