Monoplex trigonus, common name the trigonal hairy triton, is a species of predatory sea snail, a marine gastropod mollusk in the family Cymatiidae.
This species is found in the Atlantic Ocean (Cape Verde, West Africa, Gabon, Angola) and in the Caribbean Sea (Venezuela).
The shell size varies between 21 mm and 60 mm.
The maximum recorded shell length is 40 mm.
Minimum recorded depth is 15 m. Maximum recorded depth is 15 m.