Retroperitoneal primary mucinous tumor is an extremely rare malignancy. To date, only 52 cases of primary retroperitoneal mucinous cystadenocarcinoma (PRMC) have been reported in the literature, with the majority being women. Our patient is the 5th known male case. A 57-year-old male patient presented with a two year history of an undefined abdominal pain. Computed tomography demonstrated a 12 x 9.5 cm cystic mass lesion extending superiorly from right iliac fossa, while localizing below the abdominal wall and close to the psoas muscle. According to the laparotomy, iliopsoas muscle was infiltrated by the mass in the retroperitoneal region. Pathological diagnosis was mucinous cystadenocarcinoma. The asymptomatic patient was followed up without applying an adjuvant chemotherapy, since the clinical course of the tumor is known to be slow and postoperative therapy is not recognized as a standardized treatment. The clinical course of PRMC is generally indolent, however, it may manifest an aggressive course, as well. It has no definitive and clear pathogenesis. The treatment is contentious, as well. Diagnosis and treatment require surgical excision but data on adjuvant chemotherapy is limited.