Hermissenda opalescens is a species of brightly coloured sea slug or nudibranch, a marine gastropod mollusc in the family Facelinidae.

Its distribution overlaps with Hermissenda crassicornis in Northern California and this latter species is found north to Alaska.

[3] The species has also been observed in Bahia de los Angeles, Gulf of California.

This sea slug is found in various habitats, including the intertidal zone of rocky shores, but also in bays and estuaries.

[2] This nudibranch feeds on hydroids and other marine organisms such as ascidians and sea anemones.