Obesity and increased body mass index (BMI) are associated with coronavirus disease 2019 (COVID-19)-related complications and severity. They can exacerbate the cytokine storm and lead to severe symptoms or death in obese patients. This cross-sectional descriptive study included patients with COVID-19 admitted to the Razi Hospital in Ahvaz, Iran, from January 2019 to December 2020. We evaluated the effect of BMI of patients admitted to the general ward and invasive unit care (ICU) on the length of hospitalization. We included a total of 466 patients (male: 281 or 60.3% vs. female: 185 or 39.7%) with a mean age of 59.49 ± 14.5 years in the study. Also, 47 (10.1%) patients were admitted to the ICU, and 418 (89.7%) patients to the general ward. A higher BMI was associated with longer hospitalization (P < 0.001). Patients with BMI in the range of 18.5 - 24.9 experienced a longer hospitalization (10-20 days) (P < 0.001). BMI had no significant effect on ICU hospitalization (P = 0.36). Also, there was no significant difference between the two groups regarding the length of hospitalization (P = 0.49). Furthermore, non-diabetic patients were less likely to be admitted to the ICU (73.3% vs. 26.7%) (P < 0.001). The number of discharged patients was higher in patients admitted to the general ward compared to those admitted to the ICU (93.8% vs. 63.8%) (P < 0.001). According to our results, a higher BMI was a risk factor for COVID-19, especially in the early stage of infection.