Limacia janssi is a sea slug, a species of dorid nudibranch. It is a marine gastropod mollusc in the family Polyceridae.
Limacia janssi was described from Bahia Santa Elena, , Guanacaste, Costa Rica. Additional specimens from Isla Partida, , Gulf of California, Mexico were included in the original description. It has also been reported from Isla San Pedro Mártir, Gulf of California.