Objective: Prior to the coronavirus disease 2019 (COVID-19) pandemic, influenza was the most frequent cause of viral respiratory pneumonia requiring intensive care unit (ICU) admission. Few studies have compared the characteristics and outcomes of critically ill patients with COVID-19 and influenza. Methods: This was a French nationwide study comparing COVID-19 (March 1, 2020–June 30, 2021) and influenza patients (January 1, 2014–December 31, 2019) admitted to an ICU during pre-vaccination era. Primary outcome was in-hospital death. Secondary outcome was need for mechanical ventilation. Results: 105,979 COVID-19 patients were compared to 18,763 influenza patients. Critically ill patients with COVID-19 were more likely to be men with more comorbidities. Patients with influenza required more invasive mechanical ventilation (47 vs. 34%, p < 0·001), vasopressors (40% vs. 27, p < 0·001) and renal-replacement therapy (22 vs. 7%, p < 0·001). Hospital mortality was 25% and 21% (p < 0·001) in patients with COVID-19 and influenza, respectively. In the subgroup of patients receiving invasive mechanical ventilation, ICU length of stay was significantly longer in patients with COVID-19 (18 [10–32] vs. 15 [8–26] days, p < 0·001). Adjusting for age, gender, comorbidities, and modified SAPS II score, in-hospital death was higher in COVID-19 patients (adjusted sub-distribution hazard ratio [aSHR]=1.69; 95%CI=1.63–1.75) compared with influenza patients. COVID-19 was also associated with less invasive mechanical ventilation (aSHR=0.87; 95%CI=0.85–0.89) and a higher likelihood of death without invasive mechanical ventilation (aSHR=2.40; 95%CI=2.24–2.57). Conclusion: Despite younger age and lower SAPS II score, critically ill COVID-19 patients had a longer hospital stay and higher mortality than patients with influenza.