Metastasis to the lymph nodes around the iliac vessels from cancer of an unknown primary (CUP) tumor has not yet been reported in either the English or Japanese literature and it is therefore described herein for the first time. The patient was a 70-year-old woman with persistent right leg edema. Computed tomography (CT) displayed a mass around the iliac vessels while physical, laboratory, and other imaging examination did not show any other tumor. Preoperatively diagnosed as a retroperitoneal tumor, the patient underwent a tumor resection, but a histopathological examination revealed the tumor to be poorly differentiated squamous cell carcinoma. Only residual lymph nodes in the pelvis were detected by postoperative fluorine-18 fluorodeoxyglucose-positron emission tomography. Neither a primary lesion nor any signs of recurrence were demonstrated for 13 months after radiotherapy for the residual nodes. We herein discuss the diagnosis, treatment, and follow-up of this less common CUP.