Neimongosaurus is a therizinosauroid which is known from the holotype LH V0001, a partially preserved skull and skeleton, consisting of a partial braincase, the front of the right lower jaw, a nearly complete axial column, a furcula, the shoulder girdle, both humeri, a left radius, parts of the pelvis and parts of the hindlimbs. The specimen was in 1999 collected in the Sanhangobi in Inner Mongolia from the Iren Dabasu Formation dating from the late Campanian or early Maastrichtian stage, around 72-68 million years ago. A second specimen, LH V0008, consisting of a sacrum with ilia, was assigned as the paratype.
The type species, Neimongosaurus yangi, was formally named and described by Zhang Xiaohong, Xu Xing, Paul Sereno, Kwang Xuewen and Tan Lin in 2001. The generic name is derived from Nei Mongol, the Chinese name for Inner Mongolia. The specific name honours Yang Zhongjian.
Neimongosaurus was bipedal. The teeth in its deep lower jaw, coarsely serrated, indicate an herbivorous diet. It had elongated cervical vertebrae. Its scapula had a tapering end. Neimongosaurus is thought to have been about 2.3 meters in length.
Neimongosaurus was by the describers assigned to the Therizinosauroidea, in a basal position. Subsequent cladistic analyses have indicated a position in the more derived Therizinosauridae, but an analysis in 2010 by Lindsay Zanno confirmed the original placement.