© 2016 Background There is substantial evidence that exercise training improves aerobic capacity among people with multiple sclerosis (MS), but less is known about the associations between physical activity and sedentary behaviors with aerobic capacity. Objective This study examined if objectively-measured moderate-to-vigorous (MVPA) and light (LPA) physical activity and sedentary behavior were associated with peak aerobic capacity (VO2peak) measured using an established protocol for conducting a maximal, incremental exercise test in persons with MS. Methods The study involved a cross-sectional, observational study design and included 49 persons with MS. Participants wore an accelerometer around the waist during the waking hours for a 7-day period as a measure of physical activity and sedentary behaviors, and completed a maximal, incremental exercise test on an electronically-braked, computer-controlled cycle ergometer with open-circuit spirometry for measuring VO2peak. Results VO2peak was significantly correlated with MVPA (r = 0.53, p < 0.001) and LPA (r = 0.39, p < 0.01), but not sedentary behavior (r = − 0.12, p = 0.44). Linear regression analysis indicated that MVPA (B = 0.19, SE B = 0.04, β = 0.51, p < 0.001) and LPA (B = 0.02, SE B = 0.01, β = 0.30, p < 0.05), but not sedentary behavior (B = − 0.01, SE B = 0.01, β = − 0.14, p = 0.26), explained significant variance in VO2peak (R2 = 0.40). Conclusion We provide the first evidence that MVPA and LPA represent concurrent correlates of VO2peak and both could be targeted for improving aerobic capacity in persons with MS.