Retroperitoneal serous cystadenocarcinoma is an extremely uncommon lesion. Here, we present MR imaging findings of a 40-year woman who was admitted to the hospital due to abdominal pain. The patient was evaluated with abdominal Computed Tomography (CT) and Magnetic Resonance Imaging (MRI). Abdominal CT revealed a 13x18 cm large retroperitoneal and cystic mass with polypoid projections. MRI was superior than CT in showing polypoid projections. Contrast enhanced MRI detected enhancement of thin septations and cyst appearance inner the cyst. Diffusion-weighted MRI showed restricted diffusion in the polypoid component with contrast enhancement. Therefore, we thought malignant lesion. The findings of contrast-enhanced CT and MRI were helpful in the diagnosis of those lesions. In addition, diffusion weighted MRI with multi parametric modalities played unlimited role in the assesment of the differential diagnosis.