Primary retroperitoneal mucinous cystadenoma with a sarcoma-like mural nodule is a rare entity with one case reported in literature. We present a case of a 34-year-old woman who presented for evaluation of gastric pain, where a cystic abdominal mass was incidentally seen on ultrasonography. Subsequent magnetic resonance imaging (MRI) showed signal characteristics of a retroperitoneal cystic mass with hemorrhagic content and a mural nodule. After one year there was no evidence of recurrence or metastasis on follow-up ultrasonography.