Sepia plathyconchalis is a species of cuttlefish native to the western Indian Ocean, specifically the Saya-de-Malha Bank, Cargados-Carajos Shoals, and St Brandon Shoals. It lives in shallow waters at depths of between 25 and 63 m.

Sepia plathyconchalis grows to a mantle length of 8 to 63 mm.

The type specimen was collected in the Saya-de-Malha Bank, Indian Ocean (09°42′S 61°08′E). It is deposited at the Zoological Museum in Moscow.

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