Bathymodiolus thermophilus is a species of large, deep water mussel, a marine bivalve mollusc in the family Mytilidae, the true mussels. The species was discovered at abyssal depths when submersible vehicles such as DSV Alvin began exploring the deep ocean. It occurs on the sea bed, often in great numbers, close to hydrothermal vents where hot, sulphur-rich water wells up through the floor of the Pacific Ocean.