Background: Physical activity may be beneficial for people with multiple sclerosis (MS), but this population is still physically inactive compared to the general public across the disease course, even in the early stages of the disease. Research on physical activity and exercise in MS could offer solutions to promote physical activity behavior in persons with MS; however, these studies have mainly focused on adults with longer disease duration, rather than persons who have been newly diagnosed. Objectives: This study examined participant characteristics, particularly disease duration, in theory-based behavior change interventions targeting physical activity in MS and summarized theoretical frameworks and change in physical activity outcomes. Methods: PubMed, CINAHL, Embase, and Scopus were searched for identifying potential interventions. Based on predetermined eligibility criteria, one reviewer screened titles and abstracts, and two reviewers then independently screened full-text articles. Data were extracted by one reviewer and checked by a second reviewer. Results: Thirty-three trials were included in this review. One trial reported a mean disease duration of 4.5 years for the intervention group, whereas the remaining included participants diagnosed with MS for 6.7 or more years. The most common theories used were Social Cognitive Theory, Transtheoretical Model, and Motivational Interviewing. The effects on physical activity were heterogeneous; device-measured outcomes increased in 41.4% of studies, self-reported outcomes improved in 72.4%, and adherence (?80%) in 37.9% of studies. Conclusions: There is little focus on physical activity behavior change interventions in persons recently diagnosed with MS, and future research should apply comprehensive theoretical approaches for greater effects across outcome measures when targeting this and all MS sub-populations.