An aneurysmal bone cyst (ABC) is a benign tumor of the skeletal system, which is rare in childhood and mostly occurs in long bones. An aneurysmal bone cyst of the rib is also very rare and it is difficult to distinguish from other rib tumors of childhood, especially Ewing's sarcoma. An unusual case of an aneurysmal bone cyst in the rib of a 12-year-old boy is presented herein. The entity is discussed with special emphasis on the clinicopathologic features, differential diagnosis, and treatment. The most important diagnostic aid in accurately identifying such cysts is to be aware of such a possible diagnosis when a child presents with a rib mass. An en bloc resection of the mass along with the affected portion of the rib is mandatory to obtain a satisfactory outcome.