Ectopic adrenocortical tissue may be present in many anatomical localizations. Hyperplasia, adenoma or carcinoma can occasionally develop from the ectopic adrenal tissue. Therefore, it should be surgically excised when it is detected. Adrenocortical tumors are the most common type of adrenal neoplasms. Ectopic adrenocortical adenomas are rarely seen. A total of 34 cases of ectopic adrenocortical adenoma (14 of which are oncocytomas) have been reported at different localizations in English literature. Most of them are non-functional. Differential diagnosis is required with other benign or malign oncocytic neoplasms. We report a 56-year-old male patient, who presented with a retroperitoneal mass. Our case is the seventh case of ectopic retroperitoneal adrenal adenoma with oncocytic cells.