Background: Fatigue is a common symptom reported by patients with chronic immunoinflammatory diseases and with profound negative implications on health-related quality of life. This study aimed to delineate underlying components contributing to fatigue in patients with inflammatory bowel disease (IBD) receiving biologic therapy. Methods: Cross-sectional questionnaire study of all patients with IBD receiving any biologic therapy at a tertiary IBD center. Fatigue was assessed by Functional Assessment of Chronic Illness Therapy–Fatigue (FACIT-F). Disease activity and quality of life were evaluated by generic questionnaires. Principal component analysis (PCA) was used to identify components of variables explaining fatigue. Results: Three hundred patients with IBD were included. Moderate-to-severe fatigue defined as FACIT-F ≤ 39 was present in half of the included patients. PCA condensed variables associated with fatigue into three main components contributing to 49% of observed fatigue. The first component, explaining 21% of fatigue, included factors related to disease chronicity, e.g., long disease duration, high number of previously used biologic therapies, presence of previous intestinal surgery, and increasing age. The second component explained 14% of fatigue and comprised disease activity-related aspects, e.g., disease activity indices and C-reactive protein. The third explained 14% of fatigue and comprised various nutritional deficiencies. Conclusion: Fatigue can partly be explained by chronicity, disease activity, and nutritional deficits. However, the cause of fatigue is unexplained in approximately half of the patients with IBD supporting that fatigue can be an independent, systemic extraintestinal disease manifestation in IBD.