Hallopus was a prehistoric reptile, named in 1877 as a species of Nanosaurus and classified by O. C. Marsh in 1881 from the Late Jurassic Morrison Formation. Today though the animal is thought to be a pseudosuchian more closely related to crocodilians. It was redescribed as a sphenosuchian crocodylomorph in 1970, now thought to be a paraphyletic group. It was a quite small animal, reaching a length of 1 m (3.3 ft) with long and slender limbs. Macelognathus, a similarilly slender-proportioned crocodylomorph to Hallopus, may be synonymous with it.