Abstract Background Although fatigue is common in patients with inflammatory bowel disease (IBD), it often goes unrecognized and untreated. We investigated the degree of fatigue, and associated factors, in patients with IBD. Methods A multicenter study involving 147 IBD patients was conducted at five academic hospitals from August 2019 to December 2021. Demographic, clinical, and laboratory data were collected. Disease activity was assessed by the Mayo score or Crohn’s Disease Activity Index, as appropriate. Fatigue was evaluated using the validated Korean version of the Multidimensional Fatigue Inventory (MFI-K). Anxiety and depression were measured using the Hospital Anxiety and Depression Scale. Results Among 97 ulcerative colitis and 50 Crohn’s disease patients, the mean total MFI-K score was 59.0 ± 5.5. Age, sex, body mass index, disease duration, anemia, hypoalbuminemia, and use of steroids did not affect the total MFI-K score. The anxiety and depression scores did not correlate with the fatigue score. Moderate and high disease activity significantly increased the MFI-K general and physical fatigue subscale scores compared to remission and mild disease activity (17.6 ± 1.7 vs. 16.7 ± 2.0, P = 0.009), while the use of biologics significantly decreased the total MFI-K score (57.3 ± 5.0 vs. 59.5 ± 5.5, P = 0.031). Multiple linear regression showed that fatigue was significantly increased by a history of surgery for IBD and decreased by the use of biologics. Conclusion The degree of fatigue in patients with IBD is high, and disease activity is the most important contributing factor.