Older adults with multiple sclerosis (MS) experience the combined effects of aging and a chronic, disabling neurological disease on physical activity, sedentary behavior, and physical function. This study examined associations among light and moderate-to-vigorous physical activity (LPA and MVPA), sedentary behavior, and physical function in older adults with MS. Forty older adults with MS (median age = 60 years) who had a median Expanded Disability Status Scale score of 4.5 wore an accelerometer for a 7-day period and completed the Short Physical Performance Battery (SBBP), 6-minute walk (6MW), and timed 25-foot walk (T25FW). LPA was associated with SPPB (r = .551, p < 0.01), 6MW (r = .660, p < 0.01), and T25FW (r = .623, p < 0.01) scores; MVPA was associated with 6MW (r = .529, p < 0.01) and T25FW (r = .403, p < 0.01) scores. There were significant associations between LPA, but not MVPA, with SPPB (β = .583, p < 0.01), 6MW (β = .613, p < 0.01), and T25FW (β = .627, p < 0.01) scores in linear regression analyses. Older adults with MS who engaged in more LPA demonstrated better physical function and therefore LPA might be a target of future behavioral interventions.