BACKGROUND: Limited data exist on population-based risks and risk ratios (RRs) of coronavirus disease 2019 (COVID-19)-associated hospitalizations and clinical outcomes stratified by age and race/ethnicity. METHODS: Using data from electronic health records and claims from 4 US health systems for the period March 2020-March 2021, we calculated risk and RR by age and race/ethnicity for COVID-19-associated hospitalizations and clinical outcomes among adults (≥18 years). COVID-19-associated hospitalizations were defined based on COVID-19 discharge codes or a positive severe acute respiratory syndrome coronavirus 2 result. Proportions of acute exacerbations of underlying conditions were estimated among hospitalized patients with select underlying conditions, stratified by age and race/ethnicity. RESULTS: Among 2.6 million adults included in the patient cohort, 6879 had COVID-19-associated hospitalizations during March 2020-March 2021 (risk: 264 per 100 000 population). Compared with younger, non-Hispanic White adults, non-Hispanic Black and Hispanic adults aged ≥65 years had the highest hospitalization risk ratios (RR, 8.6; 95% CI, 7.6-9.9; and RR, 9.3; 95% CI, 8.5-10.3, respectively). Among hospitalized adults with COVID-19 and renal disease or cardiovascular disease, the highest proportion of acute renal failure (55.5%) or congestive heart failure (43.9%) occurred in older, non-Hispanic Black patients. Among hospitalized adults with chronic lung disease or asthma, the highest proportion of respiratory failure (62.9%) or asthma exacerbation (66.7%) occurred in older, Hispanic patients. CONCLUSIONS: During the first year of the US COVID-19 pandemic in this cohort, older non-Hispanic Black and Hispanic adults had the highest relative risks of COVID-19-associated hospitalization and adverse outcomes and, among those with select underlying conditions, the highest occurrences of acute exacerbations of underlying conditions.