Ascarosepion cultratum, commonly known as the knifebone cuttlefish or elongated cuttlefish, is a species of cuttlefish from the family Sepiidae endemic to the southern Indo-Pacific. It is a deepwater species found in subtropical and temperate Australian waters.

Naming The type specimen was collected in Twofold Bay, New South Wales and was described by William Evans Hoyle in 1885. Ascarosepion cultratum has a pale buff pinkish brown colour. It has a crescent-shaped club with a flattened sucker bearing 5 or 6 small suckers in transverse rows. The cuttlebone is an elongate oval shape with triangular pointed anterior and narrow posterior ends. It has a salmon-coloured dorsal surface with ribbing: the median rib is distinct and narrow, flanked by two faint lateral ribs. Its mantle grows to a maximum size of 12 cm.

Distribution Its Australian distribution includes waters of Queensland, New South Wales, Victoria, Tasmania, South Australia and Western Australia.

Habitat and ecology The knifebone cuttlefish typically inhabits waters between 300 and 500 m deep.