INTRODUCTION: Primary retroperitoneal mucinous cystadenoma is a rare tumor prevailing specifically in female gender. Its histogenesis is still unclear and its diagnosis is mainly based on morphological characteristics. CASE PRESENTATION: the subject is a 44 years old man presenting an abdominal pain on the right side, with a palpable mass which appeared four months ago. Abdominal ultrasound (echography) revealed a retroperitoneal cystic process, which was successfully resected through laparotomy. Histopathological examination concluded to a mucinous cystadenoma with borderline malignancy foci. After a year of follow-up, no relapse was noticed in this patient. CONCLUSION: Retroperitoneal mucinous cystadenoma is a rare tumor that should be considered in front of a retroperitoneal cystic process. Several hypotheses may explain the histogenesis of this pathological process. The interest in publishing this case report on primary retroperitoneal mucinous cystadenoma in a male patient lies in the rarity of occurrence of this syndrom in males as compared to females.