Diplodocids, or members of the family Diplodocidae ("double beams"), are a group of sauropod dinosaurs. The family includes some of the longest creatures ever to walk the Earth, including Diplodocus and Supersaurus, some of which may have reached lengths of up to 42 metres (138 ft).

Diplodocids were generally large animals, even by sauropod standards. Thanks to their long necks and tails, diplodocids were among the longest sauropods, with some species such as Supersaurus vivianae and Diplodocus hallorum estimated to have reached lengths of 30 meters (100 ft) or more. The heaviest diplodocids, such as Supersaurus and Apatosaurus, may have weighed close to 40 tonnes. However, not all diplodocids were so large; the South American species Leinkupal laticauda was one of the smallest diplodocids, with an estimated length of only 9 meters (30 ft).

Their heads, like those of other sauropods, were tiny with the nasal openings on the top of the head (though in life the nostrils themselves would have been close to the tip of the snout).

The heads of diplodocids have been widely depicted with the nostrils on top due to the position of the nasal openings at the apex of the skull. There has been speculation over whether such a configuration meant that diplodocids may have had a trunk. A 2006 study surmised there was no paleoneuroanatomical evidence for a trunk. It noted that the facial nerve in an animal with a trunk, such as an elephant, is large as it innervates the trunk. The evidence suggests that the facial nerve is very small in diplodocids. Studies by Lawrence Witmer (2001) indicated that, while the nasal openings were high on the head, the actual, fleshy nostrils were situated much lower down on the snout.

Diplodocids had long necks, which could reach an estimated length of 15 metres (50 ft) in the largest, longest-necked species. The neck was typically composed of 15 vertebrae, though in Barosaurus, the neck probably had 16 vertebrae as the result of the incorporation of an additional vertebra from the dorsal series. The habitual neck posture of diplodocids is controversial; studies have proposed postures ranging from nearly straight and below horizontal to an S-curve that reaches vertical.

A diagnosis is a statement of the anatomical features of an organism (or group) that collectively distinguish it from all other organisms. Some, but not all, of the features in a diagnosis are also autapomorphies - distinctive anatomical features that are unique to a given organism or group.

The clade Diplodocidae is distinguished based on the following characteristics:

Few skin impressions of diplodocids have been found. However, at least one significant find was reported by Stephen Czerkas in 1992. Fossils from the Howe Quarry in Shell, Wyoming preserved portions of the skin from around the tip of the tail, or "whiplash". Czerkas noted that the skin preserved a sequence of conical spines, and that other, larger spines were found scattered around larger tail vertebrae. The spines appeared to be oriented in a single row along the mid-line of the tail, and Czerkas speculated that this midline row may have continued over the animal's entire back and neck.