Semicassis bisulcata, common name: the Japanese bonnet, is a species of large sea snail, a marine gastropod mollusk in the family Cassidae, the helmet snails and bonnet snails.
The size of an adult shell varies between 25 mm and 85 mm. They are carnivorous and mainly feed on other smaller marine gastropods like Umbonium vestiarium.
This species occurs in the Red Sea and in the Indo-West Pacific.