[en] Retroperitoneal extraskeletal osteosarcoma is a rare tumor. Typical imaging findings include a soft tissue mass accompanied with internal calcifications. The authors encountered a case involving a 44-year-old woman with a large retroperitoneal extraskeletal osteosarcoma, without calcification, that mimicked a pancreatic tumor. The present report highlights computed tomography features of retroperitoneal extraskeletal osteosarcoma, followed by a brief literature review. It is challenging for radiologists to diagnose retroperitoneal masses. However, in patients who present with large retroperitoneal masses, combined with clinical information including the elevation of serum alkaline phosphatase levels, retroperitoneal extraskeletal osteosarcoma should be considered in the differential diagnosis, even if the mass does not exhibit a gross calcification on the imaging