We conducted a systematic review of observational studies to examine the effects of body mass index (BMI) and obesity (BMI ≥ 30 kg/m²) on coronavirus disease 2019 (COVID‐19). Medline, Embase, and the Cochrane Library were searched. Sixteen articles were finally included in the meta‐analysis, and a random effects model was used. BMI was found to be higher in patients with severe disease than in those with mild or moderate disease (MD 1.6, 95% CI, 0.8–2.4; p = .0002) in China; however, the heterogeneity was high (I² = 75%). Elevated BMI was associated with invasive mechanical ventilation (IMV) use (MD 4.1, 95% CI, 2.1–6.1; p < .0001) in Western countries, and this result was consistent across studies (I² = 0%). Additionally, there were increased odds ratios of IMV use (OR 2.0, 95% CI, 1.4–2.9; p < .0001) and hospitalization (OR 1.4, 95% CI, 1.3–1.60; p < .00001) in patients with obesity. There was no substantial heterogeneity (I² = 0%). In conclusion, obesity or high BMI increased the risk of hospitalization, severe disease and invasive mechanical ventilation in COVID‐19. Physicians must be alert to these early indicators to identify critical patients.