Mamenchisaurus (pronounced ma-MENCH-ih-SAW-rus) was first exposed in China by C. C. Young. It is a plant-eating four-legged dinosaur, known for its amazingly long neck.

The first specimen exposed (the type specimen) was 22 meters (72 feet) long, and half of that was neck (11 meters, or 36 feet) - which made it the longest known neck any animal at the time. 19 vertebrae were exposed (another record), along with long rods that were found in the neck.

In 1987, a different species of Mamenchisaurus was exposed (M. hochuanensis) with a neck that might have reached up to 15 meters (49 feet) in length. In 1994, the Sauroposeidon was exposed in the United States, with a neck estimated to be up to 12 meters (39 feet) long; although since the Sauroposeidon is a brachiosaurid, with very long forelimbs, it is a much taller dinosaur.

Mamenchisaurus means "Mamenchi lizard", from the Chinese ma ("horse"), men ("gate") and chi ("stream"), and the Greek sauros ("lizard"). It was named for the Mamingxi Ferry in Jinshajiang, on the Yangtze River, Yibin, in the Sichuan Province of China, where it was establish.

Most genuses lived 145 to 150 million years ago, in the Tithonian age of the late Jurassic time.

A herd of mamenchisaurs were featured in the summary scene in Jurassic Park's sequel, The Lost World: Jurassic Park.