A 36-year-old woman came to our hospital complaining of right flank pain. Computed tomographic (CT) scanning showed a cystic mass, 6 x 9 cm in size, including homogeneous low-density fluid contents, in the right retroperitoneal space. The cyst wall showed partly high-density epithelium, but there was no contrast enhancement. A 7.5 x 12 cm retroperitoneal cyst was easily removed with yellow serous fluid in it. Cytological examination showed no malignant cells in this fluid. The origin of the cyst was unknown. The histopathological diagnosis was retroperitoneal serous cyst with focal ossification in the lining epithelium. Here we report this rare case of retroperitoneal serous cyst and briefly discuss 57 cases reported in Japan.