In 1989 during a field exploration examining the outcrops at the Khongil locality in South Central Mongolia, conducted by the Mongolian and Russian Paleontological Expedition in the Gobi Desert, many dinosaur fossil discoveries were made. About away from this locality, a large and associated, but mostly disarticulated partial theropod skeleton was discovered in sediments of the Burkhant locality, Bayan Shireh Formation; no other findings were made by the expedition at this locality. It was found in fine-grained, medium sandstone/gray mudstone that was deposited dating back to the Late Cretaceous epoch. The specimen was found preserving a left maxilla with nine teeth and two empty alveoli, four cervical vertebrae, three dorsal vertebrae and eight caudal vertebrae, a nearly complete pelvic girdle compromising both pubes, right ilium and right ischium, both femora and left tibia, left metatarsals III and IV, manual and pedal phalanges with some unguals, right scapulocoracoid, an isolated radius, two ribs, and caudal chevrons. It was collected and prepared by the assistant paleontologist, Namsarai Batulseen, and stored as MNUFR-15. Ten years later, the specimen was formally described in 1999 and became the holotype for the new genus and species Achillobator giganticus. It was identified as a dromaeosaurid taxon. The description was performed by Mongolian paleontologist Altangerel Perle and North American paleontologists Mark A. Norell and James M. Clark. In terms of etymology, the generic name, Achillobator, is derived from the Latin word "Achillis" (genitive singular of Achilles) in reference to the large Achilles tendon that supported the second pedal ungual (known as "sickle claw") of most dromaeosaurids, and the old Mongolian word "баатар" (baatar, meaning hero).
