Janolus is a genus of small to large sea slugs, or more accurately nudibranchs, marine gastropod mollusks, in the family Proctonotidae. The name Janolus is derived from the two-headed god Janus, in ancient Roman mythology. Although these nudibranchs superficially resemble aeolid nudibranchs, this genus is in fact in the suborder Arminina.