Putin said there is no way to intercept the Poseidon, which analysts believe has a range of 10,000 km and can travel at about 185 km per hour.
Poseidon, known in NATO as Kanyon, is 20 metres long and weighs 100 tonnes, according to Russian media.
Arms control experts have estimated it would carry a two megaton warhead and perhaps is powered with a liquid-metal-cooled reactor.