Piatnitzkysaurus (named to honor Alejandro Mateievich Piatnitzky (1879-1959), Russian-born Argentine geologist) is a genus of theropod dinosaur. It is known from the Canadon Asfalto Formation, Middle Jurassic (Callovian stage) of what is now Argentina. Once thought to be a basal carnosaur, it may instead be a megalosauroid. Partial skeletons are known (two fractured skulls and parts of the postcranial skeleton) and show Piatnitzkysaurus was a lightly built medium-sized bipedal carnivore with robust arms around 4.3 metres (14 ft) long and around 450 kilograms (990 lb) in mass, though such estimates apply to the holotype, which is a subadult. Its ischium is 423 millimetres (16.7 in) long. Its braincase resembles that of another megalosauroid, the megalosaurid Piveteausaurus from France.
The type species, Piatnitzkysaurus floresi, was described by Jose Bonaparte in 1979.