© 2015 Human Kinetics, Inc. Background: Physical inactivity is prevalent in people with multiple sclerosis (MS) and this highlights the importance of developing behavioral interventions for increasing physical activity (PA) in MS. This pilot trial examined the efficacy of a 6-week, behavioral intervention based on social cognitive theory (SCT) delivered by newsletters and phone calls for increasing PA in persons with MS who were physically inactive and had middle levels of self-efficacy. Methods: The sample included 68 persons with relapsing-remitting MS who were randomly assigned into intervention and control groups. The intervention group received SCT-based information by newsletters and phone calls, whereas the controls received information regarding topics such as stress management over 6 weeks. Participants completed self-report of PA and social cognitive variables. Results: The intervention group had a significant increase in self-reported PA (d = 0.56, P = .02) over the 6 weeks, but the controls had a nonsignificant change (d = -0.13, P = .45). Goal setting was changed in the intervention group (d = 0.68, P ≤ .01) and identified as a significant mediator of change in self-reported PA. Conclusions: This study provides initial evidence for the benefit of a theory-based behavioral intervention for increasing PA in MS.