Background: Type 2 diabetes mellitus (DM) is associated with increased breast cancer (BC) risk and adverse BC outcomes. Less is known about the impacts of diabetes severity and its complications on BC recurrence. Our objective was to determine risk of recurrence associated with DM complications among older women with invasive BC. Methods: We conducted a retrospective cohort study of women ages 65+ years that were diagnosed with incident, invasive stages I-III BC between 2007 and 2015 using the Surveillance, Epidemiology and End Results (SEER)-Medicare Linked Database. SEER records and administrative health claims data were used to ascertain patient demographics, tumor characteristics and cancer treatment including chemotherapy, surgery and radiation. Claims data were used to estimate the prevalence of DM complications and calculate baseline and time-varying Diabetes Complications Severity Index (DCSI) scores. BC recurrences were identified using a validated algorithm for health claims data. Multivariable competing risk models accounting for death without BC recurrence were used to estimate adjusted subdistribution hazards ratios (SHR) and 95% confidence intervals (CI) for associations between DCSI scores, specific DM complications and recurrence risks. Results: Overall, 12,628 women ages 65+ years with DM at BC diagnosis were included. Over a median follow-up of 61 months, 2,134 (17%) women experienced BC recurrence. More women that developed a recurrence had DCSI scores ≥2 compared to women with DCSI score <2 (58% vs. 52%). In multivariable models, women with DCSI scores ≥3 (SHR 1.22; 95%CI 1.08-1.37) and 2 (SHR 1.15; 95%CI 1.01-1.30) had increased risks of recurrence compared to women with scores of 0. Specific macrovascular (cardiovascular disease SHR 1.10; 95% CI 1.00-1.20) and microvascular (neuropathy SHR 1.17; 95% CI 1.06-1.29) complications were independently associated with BC recurrence. Conclusions: We observed greater risk of BC recurrence in women with evidence of more severe DM and presence of macro- and microvascular complications. This association may be related to the roles of DM in BC pathogenesis through actions of insulin resistance and hyperinsulinemia on insulin-like growth factor receptors, or possibly the impacts of higher DM severity on BC treatment decision-making. Further research is warranted to investigate whether DM control and newer generation DM medications with cardiovascular benefits may improve breast cancer outcomes.