The type species is M. carinatus; seven other species have been named during the past 150 years, but only M. kaalae among these is still considered valid. Early sauropodomorphs systematics have undergone numerous revisions during the last several years, and many scientists disagree where exactly Massospondylus lies on the dinosaur evolutionary tree. The family name Massospondylidae was once coined for the genus, but because knowledge of early sauropodomorph relationships is in a state of flux, it is unclear which other dinosaurs—if any—belong in a natural grouping of massospondylids; several 2007 papers support the family's validity.
Although Massospondylus was long depicted as quadrupedal, a 2007 study found it to be bipedal. It was probably a plant eater (herbivore), although it is speculated that the early sauropodomorphs may have been omnivorous. This animal, which was 4–6 metres (13–20 ft) long, had a long neck and tail, with a small head and slender body. On each of its forefeet, it bore a sharp thumb claw that was used in defense or feeding. Recent studies indicate that Massospondylus grew steadily throughout its lifespan, possessed air sacs similar to those of birds, and may have cared for its young.
Massospondylus was a mid-size sauropodomorph, around 4 metres (13 ft) in length and weighed approximately 1000 kilograms (2200 lb), although a few sources have estimated its length at up to 6 metres (20 ft). It was a typical early sauropodomorph, with a slender body, a long neck and a proportionally very small head. The vertebral column was composed of nine cervical (neck) vertebrae, 13 dorsal (back) vertebrae, three sacral (hip) vertebrae, and at least 40 caudal (tail) vertebrae. The pubis faced forward, as with most saurischians. It had a slighter build than that of Plateosaurus, an otherwise similar dinosaur. The neck was proportionally longer than in most other plateosaurids, with the foremost cervicals being four times the length of their width. The forelimbs were only half the length of the hindlimbs but quite powerful, as indicated by the broad upper end of the humerus that provided attachment areas for a large arm musculature. Like Plateosaurus, it had five digits on each hand and foot. The hand was short and wide, with a large sickle shaped thumb claw used for feeding or defense against predators. The thumb was the longest finger in the hand, while the fourth and fifth digits were tiny, giving the forepaws a lopsided look.