Gargoyleosaurus (meaning "gargoyle lizard") is one of the earliest ankylosaurs known from reasonably complete fossil remains. Its skull measures 29 centimetres (11 in) in length, and its total body length is an estimated 3 to 4 metres (9.8 to 13.1 ft). It may have weighed as much as 1 tonne (2,200 lb). The holotype was discovered at the Bone Cabin Quarry West locality, in Albany County, Wyoming in exposures of the Upper Jurassic (Kimmeridgian to Tithonian stages) Morrison Formation.
The type species, G. parkpinorum (originally G. parkpini) was described by Ken Carpenter et al. in 1998. A mounted skeletal reconstruction of Gargoyleosaurus parkpinorum can be seen at the Denver Museum of Nature and Science. Gargoyleosaurus was present in stratigraphic zone 2 of the Morrison Formation.