Diplodocus (Greek: "double-beam") is a kind of dinosaur of subgroup Sauropoda. Diplodocus lived during the Jurassic period. Scientists gave the dinosaur its name due to the way part of its skeleton was shaped.

The first Diplodocus skeleton was establish at Como Bluff, Wyoming in 1878 and was named Diplodocus longus ("long double-beam") by paleontologist Othniel Charles Marsh. Other types include D. carnegiei (named after Andrew Carnegie) and D. hayi.

Diplodocus remains have been established in the Western United States of Colorado, Utah, Montana and Wyoming. Fossils of this animal are common, except for the skull, which is often missing from or else complete skeletons.

The skull was very small compared to the huge size of the animal, which could reach up to 27 m. Instead of the way Diplodocidae were previously portrayed, with their necks high up in the air, it is now believed by some that the animal might only keep its head very low to the ground (for grazing), and that the very long tail served as a offset for the long neck. Others think the animal could stand on its hind legs.