Bathydevius (Greek for "deep-living deviation") is a monotypic genus of unusual, highly distinctive nudibranch (sea slug) native to deep waters of the North Pacific Ocean.

It has a large, bell-shaped oral hood that it can use to catch prey, superficially similar to that of a Venus flytrap and closely resembling that of the nudibranch Melibe.

In addition, two individuals of a similar nudibranch were observed near the Mariana Trench, which would potentially extend the range of this genus or its relatives to the western Pacific.

[1][3][4] Bathydevius appears to neither sink nor rise when suspended in water, suggesting that it has a density roughly equivalent to that of seawater.

The holotype specimen was collected in January 2004, and over 157 individuals were documented in dives spanning from 2000 to 2021; during this period of study, it came to be nicknamed the "mystery mollusk" until its taxonomic identity was determined.