The genus name Elaphrosaurus is derived from the Greek words elaphro (ελαφρός) meaning "light" as in "light-weight", a reference to its slender frame and "sauros" (σαυρος) meaning "lizard"; thus, "lightweight lizard". Elaphrosaurus was described and named by Werner Janensch in 1920 and the type species is Elaphrosaurus bambergi.
Elaphrosaurus was long and slender, with a long neck. What is known about Elaphrosaurus mostly comes from a single nearly complete skeleton and no skull has been found. It was distinctive among theropods for being short-legged for its length. Paul (1988) noted that this was the longest-trunked and shallowest-chested theropod that he has examined. Elaphrosaurus was about 6.2 m (20 ft) long, 1.46 m (5 ft) tall at the hip, and weighed about 210 kilograms (463 pounds). The tibia (shin bone) of Elaphrosaurus, measured 608 mm was considerably longer than its femur (thigh bone) that measured 520 mm, which indicates that it could probably run very fast. Its long tail ended with a rare downward bend which may be unrelated to taphonomy.