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Abstract Many patients exhibit persistently reduced pulmonary diffusing capacity after coronavirus disease 2019 (COVID‐19). In this study, dual test gas diffusing capacity for carbon monoxide and nitric oxide (DL,CO,NO) metrics and their relationship to disease severity and physical performance were examined in patients who previously had COVID‐19. An initial cohort of 148 patients diagnosed with COVID‐19 of all severities between March 2020 and March 2021 had a DL,CO,NO measurement performed using the single‐breath method at 5.7 months follow‐up. All patients with at least one abnormal DL,CO,NO metric (n = 87) were revaluated at 12.5 months follow‐up. The DL,CO,NO was used to provide the pulmonary diffusing capacity for nitric oxide (DL,NO), the pulmonary diffusing capacity for carbon monoxide (DL,CO,5s), the alveolar–capillary membrane diffusing capacity and the pulmonary capillary blood volume. At both 5.7 and 12.5 months, physical performance was assessed using a 30 s sit‐to‐stand test and the 6 min walk test. Approximately 60% of patients exhibited a severity‐dependent decline in at least one DL,CO,NO metric at 5.7 months follow‐up. At 12.5 months, both DL,NO and DL,CO,5s had returned towards normal but still remained abnormal in two‐thirds of the patients. Concurrently, improvements in physical performance were observed, but with no apparent relationship to any DL,CO,NO metric. The severity‐dependent decline in DL,NO and DL,CO observed at 5.7 months after COVID‐19 appears to be reduced consistently at 12.5 months follow‐up in the majority of patients, despite marked improvements in physical performance.