Both underweight and obesity have been associated with poor prognosis in COVID-19. In an older populations of patients hospitalized for SARS-CoV-2 infection, we aimed to evaluate the association between body mass index (BMI) and short and long-term prognosis. Among 434 consecutive patients aged ≥ 70 years and hospitalized for suspected COVID-19 at a university hospital, 219 patients (median age of 83 years, 53% male) testing positive for COVID-19 and for whom BMI was recorded at admission, agreed to participate. Among them, 39 had a BMI < 20 kg/m 2 , 73 had a BMI between 20 and 24.9 kg/m 2 and 107 had a BMI ≥ 25 kg/m 2 . After adjustment for confounders, BMI < 20 kg/m 2 was associated with a higher risk of one-year mortality (hazard ratio (HR) [95% confidence interval]: 1.75 [1.00–3.05], p = 0.048), while BMI ≥ 25 kg/m 2 was not (HR: 1.04 [0.64–1.69], p = 0.9). However, BMI was linearly correlated with both in-hospital acute respiratory failure ( p = 0.02) and cardiovascular events ( p = 0.07). In this cohort of older patients hospitalized for COVID-19, low BMI, rather than high BMI, appears as an independent risk factor for death after COVID-19. The pathophysiological patterns underlying this excess mortality remain to be elucidated.