A 34-year-old female patient visited a nearby hospital with a chief complaint of right flank pain and decreased weight. Computed tomography showed a right retroperitoneal mass 10 cm in diameter on the right kidney, displacing the liver and the right kidney. The patient was referred to Kawasaki Municipal Hospital for further evaluation. The mass was suspected to be chronic expanding hematoma or neurogenic tumor of renal capsule origin. A retroperitoneal tumorectomy was performed with a right subcostal incision. A mass was noted in the smooth capsule. The mass was easily removed from the right renal capsule. However, there was significant adhesion between the mass and the peritoneum as well as the liver capsule. Therefore, a partial hepatectomy was needed for complete resection of the mass including the capsule. A fibrous capsule was noted and most of the mass was blood clot-like tissue. The histopathological diagnosis was chronic expanding hematoma with no malignancy. A retroperitoneal chronic expanding hematoma has very rarely been reported.