Background: The SpO2/FiO2 is a useful oxygenation parameter with prognostic capacity in patients with ARDS. We investigated the prognostic capacity of SpO2/FiO2 for mortality in patients with ARDS due to COVID–19. Methods: This was a post-hoc analysis of a national multicenter cohort study in invasively ventilated patients with ARDS due to COVID–19. The primary endpoint was 28–day mortality. Results: In 869 invasively ventilated patients, 28–day mortality was 30.1%. The SpO2/FiO2 on day 1 had no prognostic value. The SpO2/FiO2 on day 2 and day 3 had prognostic capacity for death, with the best cut-offs being 179 and 199, respectively. Both SpO2/FiO2 on day 2 (OR, 0.66 [95%–CI 0.46–0.96]) and on day 3 (OR, 0.70 [95%–CI 0.51–0.96]) were associated with 28–day mortality in a model corrected for age, pH, lactate levels and kidney dysfunction (AUROC 0.78 [0.76–0.79]). The measured PaO2/FiO2 and the PaO2/FiO2 calculated from SpO2/FiO2 were strongly correlated (Spearman's r = 0.79). Conclusions: In this cohort of patients with ARDS due to COVID–19, the SpO2/FiO2 on day 2 and day 3 are independently associated with and have prognostic capacity for 28–day mortality. The SpO2/FiO2 is a useful metric for risk stratification in invasively ventilated COVID–19 patients.