An articulated neck of a large, mature sauropod, with enormously elongated cervical ribs, was discovered in strata of Late Jurassic age in the eastern Junggar Basin, Xinjiang, People's Republic of China. The animal is estimated to have originally measured 26 m in length, but was lightly proportioned relative to Brachiosaurus and Apatosaurus. Rudimentary spines on the anteriormost cervical vertebrae diagnose a previously undescribed species of Mamenchisaurus. Cranial parts associated with the neck do not support a close relationship with diplodocids. Mamenchisaurs apparently belong to an endemic Jurassic radiation of Asian sauropods.