Introduction: We systematically reviewed benefits and harms of convalescent plasma (CP) in hospitalized COVID-19 patients. Material and methods: Randomized controlled trials (RCTs) and observational studies assessing CP effects on hospitalized, adult COVID-19 patients were searched until November 24, 2020. We assessed risk of bias (RoB) using Cochrane RoB 2.0 and ROBINS-I tools. Inverse variance random effect meta-analyses were performed. Quality of evidence was evaluated using GRADE methodology. Primary outcomes were all-cause mortality, clinical improvement, and adverse events. Results: Five RCTs (n = 1067) and 6 cohorts (n = 881) were included. Three and 1 RCTs had some concerns and high RoB, respectively; and there was serious RoB in all cohorts. Convalescent plasma did not reduce all-cause mortality in RCTs of severe (RR = 0.60, 95% CI: 0.33-1.10) or moderate (RR = 0.60, 95% CI: 0.09-3.86) COVID-19 vs. standard of care (SOC); CP reduced all-cause mortality vs. SOC in cohorts (RR = 0.66, 95% CI: 0.49-0.91). Convalescent plasma did not reduce invasive ventilation vs. SOC in moderate disease (RR = 0.85, 95% CI: 0.47-1.55). In comparison to placebo + SOC, CP did not affect all-cause mortality (RR = 0.75, 95% CI: 0.48-1.16) or clinical improvement (HR = 1.07, 95% CI: 0.82-1.40) in severe patients. Adverse and serious adverse events were scarce, similar between CP and controls. Quality of evidence was low or very low for most outcomes. Conclusions: In comparison to SOC or placebo + SOC, CP did not reduce all-cause mortality in RCTs of hospitalized COVID-19 patients. Convalescent plasma did not have an effect on other clinical or safety outcomes. Until now there is no good quality evidence to recommend CP for hospitalized COVID-19 patients.