Ninjemys oweni is an extinct large meiolaniid stem-turtle from Pleistocene Queensland and possibly New South Wales (Australia).

Ninjemys is primarily known from a well preserved skull and associated tail armor, which were initially thought to have belonged to the giant monitor lizard Megalania (Varanus priscus).

The remains of Ninjemys were found at the King's Creek locality in Darling Downs, Queensland, in 1879 by G. F. Bennett, an Australian collector.

Recognizing the fossil skull as that of a turtle, Bennett sent the material to the Natural History Museum, London, to the prolific paleontologist Richard Owen.

1880 saw the discovery of a fossil of a fully armored tail, later described as resembling that of a Glyptodon, which was recovered from the same spot in King's Creek that also yielded the turtle skull.

It was Thomas Henry Huxley who first identified Meiolania as a turtle in 1887 and placed the Queensland skull alongside it in the genus Ceratochelys.

However, the scales covering the head and the horns of meilaniids leave prominent marks on the underlying bone, which have been used as substitutes for the fused sutures and are deemed diagnostic for the different genera.

While Gaffney considers Niolamia a basal member of Meiolaniidae, Ninjemys and Meiolania are united by some derived features such as the presence of a second accessory ridge, the broad head and the partially separated internal nares.

[3][6] The relationship with Gaffneylania on the other hand remains uncertain, primarily due to the fragmentary nature of the later causing it to appear in several possible positions within Meiolaniidae.

[13] The two phylogenetic trees below show Ninjemys' position in Meiolaniidae as recovered by Gaffney, Archer & White (1992)[6] and Sterli, de la Fuente & Krause (2015).

Meiolania mackayi Crown Testudines Chubutemys copelloi Mongolochelys efremovi Peligrochelys walshae Otwayemys cunicularius Kallokibotion bajazidi Niolamia argentina Gaffneylania auricularis ?

[12][5] It is generally thought that meiolaniids such as Ninjemys were terrestrial animals that used their spiked bodies and clubbed tails either in intraspecific combat or to fight off predators.

Although the precise reason for the lack of known fossils is not known, it has been suggested that this could have been the consequence of Ninjemys being a generally rather rare animal in its native habitat, which currently only consists of Queensland and possibly New South Wales.