The Alfa class, Soviet designation Project 705 Lira (Russian: Лира, meaning "Lyre", NATO reporting name Alfa), was a class of nuclear-powered attack submarines in service with the Soviet Navy and later with the Russian Navy. They were the fastest military submarines ever built, with only the prototype submarine K-222 (NATO reporting name Papa-class) exceeding them in submerged speed.[3]

The Project 705 submarines had a unique design among other submarines. In addition to the revolutionary use of titanium for its hull, it used a powerful lead-bismuth cooled fast reactor as a power source, which greatly reduced the size of the reactor compared to conventional designs, thus reducing the overall size of the submarine, and allowing for very high speeds. However, it also meant that the reactor had a short lifetime and had to be kept warm when it was not being used. As a result, the submarines were used as interceptors, mostly kept in port ready for a high-speed dash into the North Atlantic.