Thespesius (meaning "wondrous one") is a dubious genus of hadrosaurid dinosaur from the late Maastrichtian-age Upper Cretaceous Lance Formation of South Dakota.

Lucas noted that Leidy's original Thespesius occidentalis fossils were indistinguishable from more complete specimens which had been referred in the late 1800s to the species Claosaurus annectens.

A wide variety of hadrosaurid species had been classified as Trachodon or "trachodonts", most notably the large "duck-billed" specimens collected by E.D.

[7][8][9] However, as early as 1913, paleontologist Lawrence Lambe regarded the type fossils of Thespesius occidentalis as inadequate and that any inferences based on them were too conjectural, as was the case for Trachodon.

A referred species of Thespesius, T. saskatchewanensis, was named by Sternberg in 1926,[10] but Nicolás Campione and David Evans found that it was a synonym of Edmontosaurus annectens in a 2011 study of edmontosaur diversity.