Orthomerus (meaning "straight femur") is a genus of dubious hadrosaurid dinosaur from the Late Cretaceous of the Netherlands.

The type species Orthomerus dolloi was in 1883 named by the well-known British paleontologist Harry Govier Seeley.

The type specimen, formed by the syntypes BMNH 42954-57, was probably found in the chalkstone quarries of the Sint-Pietersberg near the city of Maastricht, The Netherlands.

A left tibia and a metatarsal also discovered in the collection of Jacob Gijsbertus Samuël van Breda acquired by the British Museum of Natural History in 1871, were referred by him to the species.

[1] The leg bones are only about half the size of those belonging to the then largely unknown North American and Asian duckbills, with the femur 50 cm (20 in) long.

[2][3] A second species, Orthomerus weberi, was first described by Anatoly Nikolaevich Riabinin in 1945 for hindlimb elements from an unnamed Maastrichtian-age formation in the Crimea of what is now Ukraine (then a part of the Soviet Union).

However, the 2019 study included a cladistic analysis indicating that Orthomerus, though a possible hadrosaurid, was more likely placed more basal in the Hadrosauroidea, close to Gilmoreosaurus and Bactrosaurus in the evolutionary tree, outside of the Hadrosauridae.

[12] As a hadrosaurid, Orthomerus would have been a bipedal/quadrupedal herbivore, eating plants with a set of ever-replacing teeth placed in jaw bones with limited mobility that provided grinding action.