Adrenal rest is uncommon in adults, and usually represents a small lesion incidentally detected during surgery or autopsy. The adrenal rest can be detected anywhere along the path of embryonic migration of adrenal cortex, including celiac axis, genitals and broad ligament, and may be formed with the separation of cortical fragments by the migration of medullary elements from the sympathochromaffin tissue into the preformed cortical primordium. In addition, even primary adrenocortical carcinoma is a rare tumor with incidence 0.5-2 per million annually; therefore, adrenocortical carcinoma arising in adrenal rests is extremely rare. We encountered a patient with non-functioning ectopic adrenocortical carcinoma in retroperitoneum. A 34-year-old female presented with an incidentally discovered retroperitoneal mass revealed by abdominal ultrasound in her regular health examinations. She did not have any clinical abnormalities and underwent hand-assisted laparoscopic resection of the tumor. A dark-brown tumor, measuring 65×56×45 mm, was identified in the retroperitoneal space between lower pole of right kidney and inferior vena cava. Histologically, the tumor was predominantly composed of compact eosinophilic cells forming nest-like arrangements and diffusely positive for the steroidogenic factor-1. The tumor met four of the criteria of Weiss used in histological diagnosis of adrenocortical carcinoma (eosinophillic cytoplasm, nuclear atypia, atypical mitosis, and sinusoidal invasion). The tumor cells were immunohistochemically positive for 17α -hydroxylase, dehydroepiandrosterone sulfotransferase and 3β-hydroxysteroid dehydrogenase, each of which is involved in the synthesis of adrenocortical steroids. Therefore, based on these findings, we diagnosed this tumor as ectopic adrenocortical carcinoma arising in adrenal rest of retroperitoneum.