Piatnitzkysaurus (pye-at-nits-key-sore-us) is the name given to a genus of megalosaurid dinosaur. Piatnitzkysaurus lived in the Upper Jurassic, in South America. Partial skeletons have been found.

The type specimen, Piatnitzkysaurus floresi, was described by Jose Bonaparte in 1979.

Piatnitzkysaurus (meaning "Piatnitzky lizard") was a theropod dinosaur that was 14 ft (4.3 metres) long. This meat-eater had a large head, a short neck, small arms, a bulky body and clawed toes; it was similar to Allosaurus. Fossils have been found in Argentina; it is known from 2 fragmentary skeletons.

Piatnitzkysaurus was a tetanuran theropod; it lived about 169-163 million years ago. The type of species is P. floresi.