Multilocular cystic nephroma is a relatively rare benign tumor of the kidney. An adult case of spontaneously ruptured multilocular cystic nephroma was reported. A 33-year-old woman who presented with a right flank colic attack. The preoperative diagnosis was spontaneously ruptured renal tumor. A right nephrectomy was performed. A multilocular cystic lesion, 10 cm in diameter, was found in the removed specimen, which had a hematoma inside and direct tumor extension into the renal pelvis. On microscopic examination, the surface of loculi was covered by a layer of hobnail epithelium, and septa were composed of fibrous tissue with dilated vessels, lacking in normal renal components. There were no signs of malignancy. The lesion extending into the renal pelvis showed the same microscopic findings. The final diagnosis was multilocular cystic nephroma. Two respects, spontaneous rupture and tumor extension into the renal pelvis, were unique to the present case. There has been no report of spontaneously ruptured mutilocular cystic nephroma. The tumor extension into the renal pelvis, however, has been found in several reports. This growth pattern might be one of the characteristics of this benign renal tumor.