Background and objectives: Fatigue is one of the most common and disabling symptoms of multiple sclerosis (MS). It can be defined as a subjective lack of physical and mental energy. The aim of this study was to evaluate the frequency and severity of fatigue in patients with MS and its relationship with overall physical activity and disease-related disability. Materials and Methods: The study included 100 patients with a clinical relapsing-remitting form of MS. Patients with severe depression were excluded. Neurological impairment was rated using the Expanded Disability Status Scale (EDSS). Fatigue was assessed using the Modified Fatigue Impact Scale (MFIS) and the Fatigue Severity Scale (FSS), with FSS scores greater than 36 indicating patients with fatigue. Physical activity was evaluated with the International Physical Activity Questionnaire (IPAQ) and categorized on three levels: low, moderate, and high, using standard metabolic equivalents (MET). Results: The average FSS and MFIS scores were (mean ± SD) 31.3 ± 15.2 and 30.1 ± 17.0, respectively. The mean EDSS score was 2.5 ± 1.5. 42%. Patients were classified as fatigued based on FSS. Fatigued patients had higher mean EDSS scores than non-fatigued (3.0 ± 1.6 vs. 2.2 ± 1.4, respectively, p = 0.002). Low, moderate, and high levels of physical activity were reported in 35%, 20%, and 45% of patients, respectively. Higher scores of fatigue in FSS and MFIS were inversely correlated with the intensity of physical activity (r = −0.38, p < 0.001 and r = −0.33, p < 0.001, respectively). Conclusions: In patients with MS, fatigue is a common symptom. Patients with lower physical activity and greater MS-related disability have a higher severity of fatigue, which negatively affects cognitive, psychosocial, and physical functioning.