What dinosaur has the longest name?

The longest dinosaur name is Micropachycephalosaurus which is 23 letters long.

(pronounced MY-cro-PACK-ee-SEF-ah-lo-SAWR-us)

The Micropachycephalosaurus (meaning "tiny thick-headed lizard") was a very small pachycephalosaurid, a thick-skulled, plant-eating ornithischian dinosaur. It was about 1.5-3 feet (0.5-1 m) long and weighed roughly 22-33 pounds (10-15 kg). It lived during the late Cretaceous period, about 83 to 73 million years ago. Its fossils have been found in China; only a very incomplete specimen has been found.

The Micropachycephalosaurus was named in 1978 by the Chinese paleontologist Dong. The type species is M. hongtuyanensis.