A 9-year-old girl with Glenn circulation suffered from massive atelectasis of the left lung caused by bleeding during cardiac catheterization. The atelectasis resulted in frequent hypoxia leading to oxygen saturation (Spo2) of 40%–50%. In the intensive care unit, we performed a unilateral lung recruitment maneuver (ULRM) for 2 days. The ULRM involved placement of a bronchial blocker in the right main bronchus and application of continuous positive airway pressure to the left lung without hemodynamic deterioration. Eventually, Spo2 improved to 80%–85%. ULRM can be a treatment option for unilateral atelectasis in a child with Glenn circulation.