Yamatosaurus (meaning "Yamato reptile") is a genus of basal hadrosaurid from the Late Cretaceous (Maastrichtian)-aged Kita-Ama Formation of Awaji Island, Japan.

[2] Kishimoto in 2013 donated the holotype to Japan's Museum of Nature and Human Activities in the Hyogo Prefecture, where the fossils were prepared by Kazumi Wada, Tomomi Ikeda and Chisato Ota to be described in 2021.

The species Yamatosaurus izanagii was named in 2021 by Yoshitsugu Kobayashi, Ryuji Takasaki, Katsuhiro Kubota and Anthony Ricardo Fiorillo.

The fossils were found in dark grey mudstones within the upper part Kita-Ama Formation, which was deposited in marine conditions in distal turbidite facies on the edge of the continental slope in an intra-arc basin.

The sediments date to subchron 32.1r28, corresponding to the early Maastrichtian stage of the Late Cretaceous, around 71.94–71.69 million years ago, making it roughly contemporaneous with fellow Japanese hadrosaur Kamuysaurus.