Janjucetus is an extinct genus of cetacean, and a basal baleen whale (Mysticeti), from the Late Oligocene around 25 million years ago (mya) off south-east Australia, containing one species J. hunderi.

The only specimen was found on the Jan Juc beach, where the remains of the extinct whales Mammalodon, Prosqualodon and Waipatia have also been discovered.

The well-preserved fossil remains, specimen NMV P216929, include a nearly complete skull, mandibles, vertebrae, ribs, scapulae and a radius, and are held in the Museums Victoria Palaeontology Collection in Melbourne.

It had unusually large eyes for baleen whales compared to its body size, which were positioned high up on the skull; likewise, it probably relied on good eyesight instead of echolocation to navigate.

The incisors and canines formed a row of conical stabbing teeth, while the premolars and molars were shaped like serrated blades.

[1] These teeth perhaps showcase how highly specialised Janjucetus was to its niche, or indicate that it was an evolutionary dead-end given the later proliferation of baleen-bearing baleen whales.

[2] Its skull morphology seems to be convergent with the modern-day leopard seal (Hydrurga leptonyx), and so it may have used a similar grip-and-tear feeding method.

[1][7] Jan Juc Beach, where Janjucetus was discovered, also has yielded some fragmentary vertebrate species, such as sharks, rays and teleost fish.