Purpose: The Barthel Index (BI)-100 is used to measure geriatric patients’ activities of daily living (ADL). The aim of this study was to explore whether BI at hospital admission is associated with mortality. Patients and methods: In a nationwide population-based cohort study, patients aged ≥65 years admitted during 2005–2014 to Danish geriatric departments were assessed with BI at admission. Data were entered into the Danish National Database of Geriatrics and linked at the individual level to the Danish health registers (Civil Registration System, National Patient Register, and National Database of Reimbursed Prescriptions). The BI was categorized into four predefined standard subcategories according to the national Danish version of the statistical classification of diseases (BI =80–100 [independent ADL], BI =50–79 [moderate reduced ADL], BI =25–49 [low ADL], and BI =0–24 [very low ADL]). Patients were followed until death, emigration, or end of the study (December 31, 2015). Associations with mortality adjusted for age, admission year, marital status, body mass index, Charlson comorbidity index, polypharmacy, and hospitalizations during the preceding year were analyzed by multivariable Cox regression analysis. Results: Totally, 74,603 patients were included. Women (63%) were older than men (mean [SD] age; 83  vs 81  years) and had higher BI (median [IQR]; 55 [30–77] vs 52 [26–77]). Median survival (years [95% CI]) was lowest in the subcategory “BI =0–24” in both women (1.3 [1.2–1.4]) and men (0.9 [0.8–0.9]). Adjusted mortalities (HR [95% CI]; reference BI =80–100) in women were 2.41 (2.31–2.51) for BI =0–24, 1.66 (1.60–1.73) for BI =25–49, and 1.34 (1.29–1.39) for BI =50–79 and in men were 2.07 (1.97–2.18) for BI =0–24, 1.58 (1.51–1.66) for BI =25–49, and 1.29 (1.23–1.35) for BI =50–79. Conclusion: BI at admission is strongly and independently associated with mortality in geriatric patients. BI has the potential to provide useful supplementary information for the planning of treatment and future care of older patients.