The osteology of the hadrosauroid dinosaur Gilmoreosaurus mongoliensis is redescribed in detail based on the disarticulated cranial and postcranial elements of at least four individuals. These together constitute the lectotype and hypodigm of this species. The diagnosis is emended to include two autapomorphies (paddle-shaped postacetabular process that is less than 70% of the length of the iliac central plate and manual phalanx III-1 with greatly asymmetrical distal surface) and the unique combination of two iliac characters (presence of ischial tuberosity and supraacetabular process with apex located posterodorsal to ischial peduncle). The distinction of G. mongoliensis from B. johnsoni is confirmed on the basis of characters of the maxilla, dentition, ilium, ischium, and pubis. Maximum parsimony analysis places G. mongoliensis as a closely related outgroup to the Hadrosauridae, the sister taxon to the clade composed of all hadrosauroids closer to Telmatosaurus transsylvanicus than to Bactrosaurus johnsoni.