A Submarine (潜水艦 / せんすいかん , Sensuikan) is a watercraft capable of independent operation underwater. It differs from a submersible which has more limited underwater capability.

Military usage includes attacking enemy surface ships (merchant and military), aircraft carrier protection, blockade running, ballistic missile submarines as part of a nuclear strike force, reconnaissance, conventional land attack (such as using a cruise missile), and covert insertion of special forces.

Nowadays, civilian use for submarines include marine science, salvaging, exploration, and facility inspection/maintenance. Submarines can also be modified to perform more specialized functions such as search-and-rescue missions or undersea cable repair. Submarines are also used in tourism and for undersea archaeology.