Nipponosaurus (meaning "Japanese lizard") is a lambeosaurine hadrosaur from sediments of the Yezo Group, in Sinegorsk on the island of Sakhalin, which was part of Japan at the time of the species' classification.

[2][4][5] Dating the only specimen has been difficult, but based on associated mollusc taxa, the species likely lived sometime in the upper Santonian or lower Campanian, around 80 million years ago.

[4][5] The holotype (UHR 6590, University of Hokkaidō Registration) was discovered in November 1934 during the construction of a hospital for the Kawakami colliery of the Mitsui Mining Company on Karafuto Prefecture (now Sinegorsk, Sakhalin, Russia).

[4][11] Near the turn of the century, in 1994, a review of Japanese dinosaurs noted the incomplete nature of many Asian hadrosaurs could mean some of them, including Nipponosaurus, may in fact be representatives of the same species.

[4] More recently, a 2017 study provided further descriptions of the specimen, and conducted a microscopic examination on sections of the left femur, a rib, and an isolated chevron, which re-affirmed the age of the holotype.

[4] Later, in 2017, another team re-evaluated the specimen, and determined through investigating the fossils at a microscopic level (in a process called histology) that this was correct, although they doubted the significance of some of the characters identified in the former study.

[2][4] Some authors have deemed the species a nomen dubium, questioning its supposed diagnostic characteristics, especially in light of its presumed immature status.

[4] In their description of Sahaliyania and Wulagasaurus, Pascal Godefroit and colleagues reported based on direct observation of the type specimen that all of these supposed unique traits were found in other hadrosaurids.

[4] However, later in 2007, a study providing a redescription of Lambeosaurus magnicristatus re-evaluated a character relating to the size of the distal ischial foot, and found this drew Nipponosaurus away from H. altispinus and into a more basal position within their Corythosaurini (now generally referred to as the Lambeosaurini).

They however noted the only character uniting it specifically with this group, an increased number of cervical vertebrae, was a state poorly known in more basal Asian lambeosaurines, and cautioned its position was likely liable to change.

However, Nagao noted fossil molluscs now called Parapuzosia japonica and Sphenoceramus schmidti were also found at the locality, and these are known to hail from the lower Campanian.