Sepia plana is a species of cuttlefish native to the coast of Australia, specifically the North West Shelf (12°04′S 122°59′E / 12.067°S 122.983°E / -12.067; 122.983 to 17°55.5′S 118°16.0′E / 17.9250°S 118.2667°E / -17.9250; 118.2667). However, cuttlebones of this species have also been found off eastern Australia, suggesting that it may have a wider distribution. S. plana lives at depths of between 396 and 505 m.
S. plana exhibits sexual dimorphism, as females grow considerably larger than males. They reach a mantle length of 151 mm and 99 mm, respectively.
The type specimen was collected in the North West Shelf, western Australia (14°15′S 121°59′E / 14.250°S 121.983°E / -14.250; 121.983). It is deposited at the Museum of Victoria in Melbourne.
