Russia's first regiment of Avangard hypersonic missiles has been put into service, the defence ministry says.
The location was not given, although officials had earlier indicated they would be deployed in the Urals.
President Vladimir Putin has said the nuclear-capable missiles can travel more than 20 times the speed of sound and put Russia ahead of other nations.
They have a "glide system" that affords great manoeuvrability and could make them impossible to defend against.
Defence Minister Sergei Shoigu confirmed the "Avangard hypersonic glide vehicle entered service at 10:00 Moscow time on 27 December", calling it a "landmark event".
Mr Putin said on Tuesday the Avangard system could penetrate current and future missile defence systems, adding: "Not a single country possesses hypersonic weapons, let alone continental-range hypersonic weapons."
The West and other nations were "playing catch-up with us", he said.
Mr Putin unveiled the Avangard and other weapons systems in his annual state-of-the-nation address in March 2018, likening it to a "meteorite" and a "fireball".
In December 2018, the weapon hit a practice target 6,000km (3,700 miles) away in a test launch at Dombarovskiy missile base in the southern Ural Mountains.
"The Avangard is invulnerable to intercept by any existing and prospective missile defence means of the potential adversary," Mr Putin said after the test.
Mounted on top of an intercontinental ballistic missile, the Avangard can carry a nuclear weapon of up to two megatons. Russia's defence ministry has released video of the Avangard system, but weapons experts have expressed scepticism about its effectiveness.
In a statement, the Pentagon said it would "not characterise the Russian claims" about the Avangard's capabilities. The US has its own hypersonic missile programme, as does China, which in 2014 said it had conducted a test flight of such as weapon.
On 26 November Russia allowed US experts to inspect the Avangard under the rules of the 2010 New START treaty, an agreement that seeks to reduce the number of strategic nuclear missile launchers.
The New START accord, which expires in February 2021, is the last major nuclear arms control treaty between Russia and the US.
In August this year, the US withdrew from the Intermediate-Range Nuclear Forces Treaty (INF), which was signed by US President Ronald Reagan and Soviet leader Mikhail Gorbachev in 1987.
US President Donald Trump said he wanted a new nuclear pact to be signed by both Russia and China.
It is hard to determine if Russia's new Avangard hypersonic missile system really has entered service, as Moscow claims, or if this is just an advanced phase of field testing.
But President Putin's eagerness to claim bragging rights is to some extent justified. Russia looks to be ahead in the hypersonic stakes. China is also developing such systems; while the US appears to be somewhat behind.
Hypersonic missiles, as their name implies, fly very fast, at above Mach 5 - ie at least five times the speed of sound.
They can be cruise-type missiles, powered throughout their flight. Or, they can be carried aloft on board a ballistic missile from which the hypersonic "glide vehicle" separates and then flies to its target.
Such "boost-glide" systems, as they are known (Avangard appears to be one of these), are launched like a traditional ballistic missile, but instead of following an arc high above the atmosphere, the re-entry vehicle is put on a trajectory that allows it to enter Earth's atmosphere quite quickly, before gliding, un-powered, for hundreds or thousands of kilometres.
It is not so much the speed of the hypersonic weapon alone that counts. It is its extraordinary manoeuvrability as it glides towards its target.
This poses a huge problem for existing anti-missile defence systems.
Indeed the glide vehicle's trajectory, "surfing along the edge of the atmosphere" as one expert put it to me recently, presents any defensive system with additional problems.
Thus, if Russia's claims are true, it has developed a long-range intercontinental missile system that may well be impossible to defend against.
The announcement that Avangard is operational heralds a new and dangerous era in the nuclear arms race.
It confirms once again President Putin's focus on bolstering and modernising Russia's nuclear arsenal. It's indicative of the return of great power competition.
Some analysts might well see Russia's development programme as a long-term strategy to cope with Washington's abiding interest in anti-missile defences. The US argument that these are purely designed to counter missiles from "rogue-states" like Iran or North Korea has carried little weight in Moscow.
This all comes at a time when the whole network of arms control agreements inherited from the Cold War is collapsing.
One crucial treaty - the New START agreement - is due to expire in February 2021. Russia seems willing to extend the agreement but the Trump administration has so far appeared sceptical.
With a whole new generation of nuclear weapons at the threshold of entering service, many believe not just that existing agreements should be bolstered, but that new treaties are needed to manage what could turn into a new nuclear arms race.