INTRODUCTION: Although both obesity and coronavirus disease 2019 (COVID-19) independently induce inflammation and thrombosis, the association between obesity class and risk of thrombosis in patients with COVID-19 remains unclear. METHODS: This retrospective cohort study included consecutive patients hospitalized with COVID-19 at a single institution. Patients were categorized based on obesity class. The main outcomes were venous thromboembolism (VTE) and myocardial injury, a marker of microvascular thrombosis in COVID-19. Adjustments were made for sociodemographic variables, cardiovascular disease risk factors and comorbidities. RESULTS: 609 patients with COVID-19 were included. 351 (58%) patients were without obesity, 110 (18%) were patients with class I obesity, 76 (12%) were patients with class II obesity, and 72 (12%) were patients with class III obesity. Patients with class I and III obesity had significantly higher risk-adjusted odds of VTE compared to patients without obesity (OR = 2.54, 95% CI: 1.05-6.14 for class I obesity; and OR = 3.95, 95% CI: 1.40-11.14 for class III obesity). Patients with class III obesity had significantly higher risk-adjusted odds of myocardial injury compared to patients without obesity (OR = 2.15, 95% CI: 1.12-4.12). Both VTE and myocardial injury were significantly associated with greater risk-adjusted odds of mortality. CONCLUSION: This study demonstrates that both macrovascular and microvascular thromboses may contribute to the elevated morbidity and mortality in patients with obesity and COVID-19.