Background Rehabilitation therapy for patients with severe coronavirus disease 2019 (COVID-19) is crucial; however, studies regarding rehabilitation strategies for intensive care unit (ICU) patients with COVID-19 are limited. We report a case of severe COVID-19 in an ICU patient whose physical function and basic movement ability rapidly improved after the initiation of active aerobic exercise in the supine position.Case A 70-year-old man with hypertension and obesity was admitted to the ICU and managed with a ventilator because of severe COVID-19. Physical therapy started on postadmission day 34. Problems encountered during physical therapy included low saturation of percutaneous oxygen (SpO2; <90%), dyspnea with a light exercise load, advancing muscle weakness, and endurance decline. The rehabilitation program included getting out of bed and resistance training of the upper and lower limbs twice daily while maintaining SpO2 at ≥90%. After ventilator weaning, we initiated aerobic training using a supine ergometer with varying load volume. On discharge from the ICU on postadmission day 45, the patient' s physical function (handgrip strength, Medical Research Council score, and Borg scale) and basic movement ability (Functional Status Score for ICU) rapidly improved.Conclusion Rehabilitation therapy involving aerobic cycling training based on a quantitative load setting may be effective in treating COVID-19.