Palaeosaniwa was roughly comparable to a large monitor lizard (Varanidae) in size. Measuring perhaps five or six feet in length, it is the largest terrestrial lizard known from the Mesozoic period. It is similar to modern varanid lizards (particularly the Komodo dragon) in having bladelike teeth with minute serrations. These teeth would have been effective for seizing and cutting large prey items, and suggest that Palaeosaniwa fed on other vertebrates. Adult Palaeosaniwa would have been large enough to prey on any of the birds or mammals known from the time, small dinosaurs, and the eggs and juveniles of large dinosaurs.