Little is known about the complications of COVID-19. What happens when a patient with carbon monoxide poisoning has complications from COVID-19 pneumonia (a disease that has emerged since 2019 and quickly caused a pandemic around the world)? We report a case of carbon monoxide poisoning with COVID-19 pneumonia resulting in secondary pneumomediastinum. The patient underwent systemic hyperbaric oxygen therapy for two weeks and most of his symptoms improved. This case illustrates the success of hyperbaric oxygen in the simultaneous treatment of COVID-19 pneumonia, pneumomediastinum, and carbon monoxide poisoning.