| Named By: | Othniel Charles Marsh in 1879 |
| Time Period: | Late Jurassic, 155-151 Ma |
| Location: | USA - Morrison Formation deposits in Wyoming and Utah |
| Size: | B. excelsus and B. parvus estimated to be about 22 meters long. B. yahnahpin estimated to be about 21 meters long |
| Diet: | Herbivore |
| Fossil(s): | Several individuals including some fossils of juveniles |
| Classification: | | Chordata | Reptilia | Dinosauria | Saurischia | Sauropoda | Diplodocidae | Apatosaurinae | |
| Also known as: | | Elosaurus | Eobrontosaurus | |
Brontosaurus ( bron-ta-SAWR-as), meaning "thunder lizard" (from Greek bronte, bronte = thunder + sauros, sauros = lizard), is a genus of gigantic quadruped sauropod dinosaurs. Although the type species, B. excelsus, had long been considered a species of the closely related Apatosaurus, more recent research has proposed that Brontosaurus is a genus separate from Apatosaurus that contains three species: B. excelsus, B. yahnahpin, and B. parvus.
Brontosaurus had long, thin necks and small heads, adapted for a herbivorous lifestyle; a bulky, heavy torso; and long, whip-like tails. The various species lived during the Late Jurassic epoch in the Morrison Formation of North America, going extinct by the end of the Jurassic. Adult individuals of Brontosaurus are estimated to weigh up to 15 tonnes (15 long tons; 17 short tons) and measure up to 22 metres (72 ft) long; this places Brontosaurus among the largest land animals along with other diplodocids.
As the archetypal sauropod, Brontosaurus is one of the best-known dinosaurs, and has been featured in film, advertising, and postal stamps, as well as many other types of media.