Congenital thoracic ectopic kidney is a very rare developmental anomaly and the rarest form of all ectopic kidneys. It is usually asymptomatic and discovered incidentally on a routine chest radiography. We report a thoracic ectopic kidney in a 19-month-old boy, which initially presented as a well demarcated mass at the base of the right lung on chest x-ray. Intravenous pyelography (IVP) and thoraco-abdominal computed tomography (CT) demonstrated a normal functioning transdiaphragmatic thoracic ectopic right kidney, but technetium-99m DTPA and DMSA scintigraphy demonstrated pelvic stasis. We hereby discuss the features of congenital thoracic ectopic kidney and review the literature. Although it is extremely rare, thoracic ectopic kidney should be considered in differential diagnosis of a mass with a well demarcated superior margin in the lower part of the thorax, and renal scintigraphy must be performed even if CT and IVP results are normal.