Very little direct evidence exists on use of corticosteroids in patients with coronavirus disease 2019 (COVID-19). Indirect evidence from related conditions must therefore inform inferences regarding benefits and harms. To support a guideline for managing COVID-19, we conducted systematic reviews examining the impact of corticosteroids in COVID-19 and related severe acute respiratory illnesses. We searched standard international and Chinese biomedical literature databases and prepublication sources for randomized controlled trials (RCTs) and observational studies comparing corticosteroids versus no corticosteroids in patients with COVID-19, severe acute respiratory syndrome (SARS) or Middle East respiratory syndrome (MERS). For acute respiratory distress syndrome (ARDS), influenza and community-acquired pneumonia (CAP), we updated the most recent rigorous systematic review. We conducted random-effects meta-analyses to pool relative risks and then used baseline risk in patients with COVID-19 to generate absolute effects. In ARDS, according to 1 small cohort study in patients with COVID-19 and 7 RCTs in non-COVID-19 populations (risk ratio [RR] 0.72, 95% confidence interval [CI] 0.55 to 0.93, mean difference 17.3% fewer; low-quality evidence), corticosteroids may reduce mortality. In patients with severe COVID-19 but without ARDS, direct evidence from 2 observational studies provided very low-quality evidence of an increase in mortality with corticosteroids (hazard ratio [HR] 2.30, 95% CI 1.00 to 5.29, mean difference 11.9% more), as did observational data from influenza studies. Observational data from SARS and MERS studies provided very low-quality evidence of a small or no reduction in mortality. Randomized controlled trials in CAP suggest that corticosteroids may reduce mortality (RR 0.70, 95% CI 0.50 to 0.98, 3.1% lower; very low-quality evidence), and may increase hyperglycemia. Corticosteroids may reduce mortality for patients with COVID-19 and ARDS. For patients with severe COVID-19 but without ARDS, evidence regarding benefit from different bodies of evidence is inconsistent and of very low quality. © 2020 Joule Inc. or its licensors.