BACKGROUND: Abnormal red blood cell distribution width (RDW) is associated with poor cardiovascular, respiratory, and coronavirus disease 2019 (COVID-19) outcomes. However, whether RDW provides prognostic insights regarding COVID-19 patients admitted to the intensive care unit (ICU) was unknown. Here, we retrospectively investigated the association of RDW with 30-day and 90- day mortalities, duration of mechanical ventilation, and length of ICU and hospital stay in patients with COVID-19. METHODS: This study included 321 patients with COVID-19 aged >18 years who were admitted to the ICU between March 2020 and July 2022. The outcomes were mortality, duration of mechanical ventilation, and length of stay. RDW >14.5% was assessed in blood samples within 24 hours of admission. RESULTS: The mortality rate was 30.5%. Multivariable Cox regression analysis showed an association between increased RDW and 30-day mortality (hazard ratio [HR], 3.64; 95% CI, 1.54-8.65), 90-day mortality (HR, 3.66; 95% CI, 1.59-8.40), and shorter duration of invasive ventilation (2.7 ventilator-free days, P=0.033). CONCLUSIONS: Increased RDW in COVID-19 patients at ICU admission was associated with increased 30-day and 90-day mortalities, and shorter duration of invasive ventilation. Thus, RDW can be used as a surrogate biomarker for clinical outcomes in COVID-19 patients admitted to the ICU.