Mussaurus (moos-SORE-us) is an plant-eating dinosaur known from 18 to 37 cm long (9 to 16 in) fossilized juvenile and infant skeletons, which are the smallest dinosaur skeletons known.

Mussaurus is a very early dinosaur, dating from the Late Triassic Period about 215 million years ago. The infant skeleton was small, about the size of a small lizard. The juveniles had short heads and necks, long tails, and large eye sockets. Adults had longer necks, and probably reached 3 metres (10 ft) in length, and weighed 120 kg (260 lb).