A new species of the family Tritoniidae is described for the tropical South Atlantic Ocean. The animal was found off north-east Brazil. Marionia limceana sp. nov. is up to 31 mm long, with a sturdy white body, of which the notum is covered with two rows of red polygons running from the rhinophores to the tail; notum with tubers; bi-lobed veil with 16 velar papillae; retractable white rhinophores; 11–14 pairs of branchial plumes; the anus is located below the 4th gill on the right side, and the genital opening is under the 3rd gill. Internally, M. limceana sp. nov. is distinguished from other tritoniids by jaws with three or four rows of denticles on the inner lips, a belt of 18 stomach plates; and the radular formula 26 × 26–126.96.36.199.26–32 teeth. The animal was found feeding on octocorals of a species of the new genus Stragulum, and is the first reported nudibranch to feed on this genus.