Pneumomediastinum, a collection of mediastinal air, often results from the rupture of intrathoracic structures. A 41-year-old diabetic woman initially presented with signs of pneumomediastinum and nuchal subcutaneous emphysema, but was finally diagnosed with unilateral emphysematous pyelonephritis. Pneumomediastinum as a presentation in retroperitoneal infection has not been reported previously, which prompts us to discuss its etiology and emphasize the importance of physical examination.