K-222 was the sole Project 661 "Anchar" nuclear-powered cruise-missile submarine of the Soviet Navy during the Cold War. Although the Soviets saw K-222 as an unsuccessful design, upon completion it was the world's fastest submarine and the first to be built with a titanium hull.

A cruise missile submarine is a submarine that carries and launches cruise missiles as its primary armament. Missiles greatly enhance a warship's ability to attack surface combatants and strike land targets; although torpedoes are a more discrete option for submerged submarines, missiles give a much longer stand-off range, shorter time to impact the target, as well as the ability to engage multiple targets on different headings at the same time. Many cruise missile submarines retain the capability to deploy nuclear warheads on their missiles, but they are considered distinct from ballistic missile submarines due to the substantial differences between the two weapons systems' flight characteristics; cruise missiles fly aerodynamically using flight surfaces like wings or fins, while a ballistic missile uses its engine power alone as it may exit the atmosphere.