Antonietta janthina is a colourful species of sea slug or aeolid nudibranch, a marine gastropod mollusc in the family Facelinidae.
This nudibranch is known from south-east Japan. For decades, a similarly coloured species found in Indonesia, the Philippines and Thailand was thought to be a tropical variant, however it is now known to be the separate species Trinchesia nakapila.
Antonietta janthina has a whitish body with white surface pigment. The numerous cerata have a blue digestive gland inside them and superficial yellowish or white pigment covering the tips. The oral tentacles are bright orange-yellow and there is a median band of the same colour on the head. The rhinophores are scarlet with a median band of white.