Mamenchisaurus, meaning "Mamen brook lizard," is a genus of sauropod dinosaur belonging to the family Mamenchisauridae. It lived approximately 160 to 145 million years ago in what is now China. The first fossils of Mamenchisaurus were discovered in 1952 in Sichuan Province, China. Since then, numerous well-preserved specimens have been discovered of this colossal dinosaur.

Mamenchisaurus is renowned for its exceptionally long neck, which is often estimated to measure up to 9 meters in length, comprising over half of its total body length. The neck was comprised of numerous elongated vertebrae that allowed for great flexibility and the ability to reach vegetation at various heights. The rest of its body was relatively large, reaching lengths of up to 25 meters. Its body was supported by robust limbs and ended with a long, whip-like tail. The skull of Mamenchisaurus was small in proportion to its massive body, featuring peg-like teeth adapted for stripping leaves and branches.