Abstract Intra-abdominal (as opposed to extremity or limb and limb-girdle) soft tissue sarcomas (STS) are rare and account for less than 1% of all diagnosed neoplasms. These tumours are usually associated with a poor prognosis and are often locally invasive and metastatic at the time of presentation. Retroperitoneal sarcomas with synchronous or metachronous different histological types are rare and intra-duodenal sarcomas extremely unusual. A case of a giant retroperitoneal STS weighing approximately 15 kg consisting of two histologically different types is presented. Intra-duodenal involvement with sarcoma was found intra-operatively. We discuss the management of this condition in the context of an illustrative case in our recent experience.