A bathyscaphe () is a free-diving, self-propelled deep-sea submersible, consisting of a crew cabin similar to a Bathysphere, but suspended below a float rather than from a surface cable, as in the classic Bathysphere design.

The float is filled with gasoline because it is readily available, buoyant, and, for all practical purposes, incompressible. The incompressibility of the gasoline means the tanks can be very lightly constructed, since the pressure inside and outside the tanks equalises, eliminating any differential. By contrast, the crew cabin must withstand a huge pressure differential and is massively built. Buoyancy at the surface can be trimmed easily by replacing gasoline with water, which is denser.

Auguste Piccard, inventor of the first bathyscaphe, composed the name bathyscaphe using the Ancient Greek words βαθύς (bathús), meaning "deep", and σκάφος (skáphos), meaning "vessel, ship").