| Named By: | Edward Drinker Cope in 1872 |
| Time Period: | Late Cretaceous, 66 Ma |
| Location: | USA, Wyoming - Lance Formation |
| Size: | Roughly estimated at ten meters long, but this estimate is far from certain due to lack of overall remains |
| Diet: | Herbivore |
| Fossil(s): | Partial post cranial remains including dorsal (back), sacral and caudal (tail) vertebrae (16 in total), partial pelvis and a few ribs |
| Classification: | | Chordata | Reptilia | Dinosauria | Ornithischia | Ceratopsidae | Chasmosaurinae | Triceratopsini | |
Agathaumas ("great wonder") is a dubious genus of a large ceratopsid dinosaur that lived in Wyoming during the Late Cretaceous (late Maastrichtian stage, 66 million years ago). The name comes from Greek, agan - 'much' and thauma - 'wonder'. It is estimated to have been 9 metres (30 ft) long and weighed 6 tonnes (5.9 long tons; 6.6 short tons), and was seen as the largest land animal known at the time of its discovery.
It was the first named ceratopsian, though relatively little is known about it. The original specimen consisted only of the animal's hip bones, hip vertebrae and ribs, and because these bones vary little between ceratopsid species, it is usually considered a nomen dubium. It is provisionally considered a synonym of Triceratops, but is difficult to compare to that genus because it is only known from postcranial remains.