Purpose To quantify the impact of diabetes on the trajectory of cancer-related fatigue (CRF) from pre-chemotherapy to 6 months post-chemotherapy for patients with breast cancer compared to non-cancer controls.Methods This was a secondary analysis from a nationwide prospective longitudinal study of female patients with breast cancer undergoing chemotherapy and age-matched women without cancer (controls). CRF was measured using the Multidimensional Fatigue Symptom Inventory (MFSI) pre-, post-, and 6-months post-chemotherapy in patients; controls were assessed at equivalent time points. Diabetes status was obtained at baseline. Repeated measures mixed models estimated the association between CRF and diabetes controlling for cancer (y/n), body mass index, exercise and smoking habits, baseline anxiety and depressive symptoms, menopausal status, marital status, race, and education.Results A total of 439 patients and 235 controls (age: 52.8±10.5 years) had available data on diabetes status. Diabetes was twice as prevalent among patients as controls (11.6% vs. 6.8%). Patients had worse fatigue than controls throughout treatment (p<0.001). Diabetes was associated with worse CRF with a clinically meaningful difference of 4.7±1.7 points on the fatigue measure in all participants (p=0.009) and patients alone (p=0.030). For MFSI subdomains, diabetes was associated with worse general (p=0.002), physical (p=0.005), and mental fatigue (p=0.025) but not worse emotional fatigue or vigor (p>0.14) among patients. Conclusions Diabetes was twice as prevalent in women with breast cancer compared to controls, and diabetes was associated with more severe CRF in patients before and after chemotherapy and at 6 months post-chemotherapy. Interventions that address diabetes management may also help address CRF during chemotherapy treatment