Primary carcinoid tumor, especially that arising in a mature teratoma of the kidney, is extremely rare; only 3 cases have been reported in the world literature to date. Because of the rarity of the lesion, its histogenesis and prognosis are unknown. We report a case of primary renal carcinoid tumor occurring in a mature teratoma in a 30-year-old woman. A computed tomographic scan of the abdomen revealed a mass in the left kidney containing dense calcification with minimal contrast enhancement. Histologically, the tumor was composed of trabecular and anastomosing ribbonlike nests, identical to the features of carcinoid tumors of other sites. Immunohistochemical stainings were positive for cytokeratin, neuron-specific enolase, and chromogranin. In addition, there were mature teratoid tissues, such as columnar epithelium, smooth muscle, and bone. The carcinoid tumor was under and closely apposed to the lining of the cysts. The patient did not have clinical manifestations of the carcinoid syndrome and had an uneventful recovery.