Fatigue is a common symptom in patients with rheumatoid arthritis (RA) and in patients with cancer (CA). The aim was to investigate the degree of fatigue in RA patients as compared to CA patients as well as potential influencing factors on RA-related fatigue. This was a retrospective analyses of two prospective cohort studies that used the EORTC QLQ-FA12 as a common instrument to assess fatigue. The cohort of RA patients was based on a nationwide survey in Germany. The cohort of CA patients was recruited in the context of an international validation field study. Multivariable ANCOVAs compared levels of fatigue between the two cohorts, also including various subgroup analyses. Regression analyses explored influencing factors on RA patients' fatigue. Data of n = 705 RA patients and of n = 943 CA patients were available for analyses. RA patients reported significantly higher Physical Fatigue (mean difference = 7.0, 95% CI 4.2-9.7, p < 0.001) and Social Sequelae (mean difference = 7.5, 95% CI 4.7-10.2, p < 0.001). CA patients reported higher Cognitive Fatigue (mean difference = 3.5, 95% CI 1.4-5.6, p = 0.001). No differences in Emotional Fatigue (p = 0.678) and Interference with Daily Life (p = 0.098) were found. In RA patients, mental health and pain were associated with fatigue (p values < 0.001). RA patients showed a considerable level of fatigue that is comparable to and in certain cases even higher than that of CA patients. The implementation of standardized diagnostic procedures and interventions to reduce fatigue in RA patients are recommended.