Background: Fatigue is one of the most common and disabling symptoms of multiple sclerosis (MS). Participation in physical activity is reported to decrease symptoms associated with MS. However, studies consistently find that individuals with MS are less physically active when compared to general healthy population. Objectives: To assess the relation between lifestyle physical activity and perceived physical and cognitive fatigue in individuals with relapsing remitting MS (RRMS). Methods: We performed cross-sectional analyses on data collected during the pre-intervention observation phase of a dietary intervention study. To measure lifestyle physical activity, participants wore a wrist-worn accelerometer for 7 days. Perceived fatigue was measured via questionnaires: 1) Fatigue Severity Scale (FSS), 2) Modified Fatigue Impact Scale (MFIS)-physical, -cognitive and -psychosocial subscales and 3) Fatigue Scale for Motor and Cognitive Functions (FSMCF)- cognitive and motor subscales. We examined associations between physical activity and measures of fatigue using general linear models. Each model included an activity variable (light, moderate, vigorous intensity) and considered several control variables such as age, gender, years since MS diagnosis, Healthy Eating Index score, distance walked during 6-minute walk test, body mass index, vitamin D intake etc. Model ?ts were compared using the Akaike information criterion to identify the statistically optimal set of controls for each outcome-activity association. Comparisons with p-values ? 0.05 were considered significant. Results: Participants were 83 individuals with RRMS, 85.5% females, mean (SD) age of 45.4 (9.6) years and had MS for 10.7 (8.2) years. On average, participants were performing light intensity activities for 1295 (421) minutes per week, moderate intensity activities for 368 (268) minutes per week and vigorous intensity activities for 12 (56) minutes per week. While controlling for demographic and clinical variables, we found that higher duration of moderate intensity activity was associated with lower FSMCF-cognitive (p = 0.0137) and FSMCF-motor scores (p = 0.0241) and lower FSS scores (p = 0.0011). Conclusions: Our results emphasize the importance of moderate intensity physical activity to reduce perceived physical and cognitive fatigue in individuals with RRMS and provide support for using moderate intensity interventions to reduce MS-related fatigue.