Postoperative atelectasis should be avoided in surgical patients with impaired pulmonary function. Nasal high-flow (NHF) therapy delivered by the Optiflow™ system (Fisher & Paykel Healthcare Ltd., Auckland, New Zealand) is a new, simple device that supplies heated and humidified oxygen gas at >30 L/min via a large-bore nasal cannula. We herein describe a case in which respiratory support with NHF therapy was useful for the prevention of postoperative atelectasis recurrence. A 67-year-old man with an upper digestive tract perforation underwent emergency laparoscopic surgery. He appeared malnourished because of severe untreated diabetes mellitus. The proposed surgery was uneventfully completed. On postoperative day (POD) 5, he experienced massive atelectasis of the left lower lobe with desaturation to <90%. After restoration of normal oxygenation by tracheal suction and postural drainage, noninvasive positive-pressure ventilation (NPPV) at a continuous positive airway pressure (CPAP) of 8 cm H2O was conducted to prevent repeated atelectasis. Fifteen hours after the cessation of NPPV on POD 7, he developed recurrence of massive atelectasis. Bronchoscopic suction removed a mucous plaque in the tracheobronchial tree, and NHF therapy at 40 L/min was subsequently performed, delivering a low level of CPAP instead of NPPV. Under the respiratory support with NHF therapy, his condition was more stable than with NPPV, and his respiratory rehabilitation continued uneventfully. In addition, the NHF therapy delivered optimally humidified gas, which improved the bronchial secretion quality. No further atelectasis occurred throughout the remaining stay in the intensive care unit. We conclude that respiratory support with NHF therapy may contribute to the prevention of postoperative atelectasis by delivering CPAP in combination with progressive respiratory rehabilitation.