A seven-year-old, castrated male, Korean domestic shorthair cat was referred because of a kidney abnormality. Radiography revealed left renal agenesis and right kidney enlargement. Ultrasonography and computed tomography (CT) showed amorphous calcified materials in a cyst-like lesion of the right kidney. In the excretory phase of the CT images, the lesion was opacified with contrast materials, indicating communication with the collecting system. Based on these findings, the cat was diagnosed with a calyceal diverticulum. A calyceal diverticulum may cause various clinical symptoms related to the urinary system. The excretory phase of CT is useful for diagnosing a calyceal diverticulum.