"South Korean and US intelligence are closely analysing for additional detail," it added.
Sinpo, where the missile was fired, is a major naval shipyard and satellite photographs have previously shown submarines at the facility.
The North is known to be developing a submarine-launched ballistic missile (SLBM) and previously carried out an underwater launch, although analysts said that one was likely to have been from a submerged platform rather than a submarine.
Something of a regional arms race is developing on the peninsula, with the South last month testing its first SLBM, putting it among the elite group of nations that have demonstrated proven technology, and unveiling a supersonic cruise missile.
Following Tuesday's launch, the South's presidential office said it was convening a meeting of the National Security Council.
The nuclear-armed North -- which invaded its neighbour in 1950 -- has in recent weeks tested a long-range cruise missile, a train-launched weapon and what it said was a hypersonic warhead, sparking global concern.
It also mounted a rare weapons exhibition, showcasing the gigantic international ballistic missile (ICBM) revealed at a night-time military parade last year.
Opening the event, leader Kim Jong Un -- who has overseen rapid progress in the North's military technology, at the cost of international sanctions -- blamed the United States for tensions, dismissing Washington's assertions that it does not have hostile intentions.
'No hostile intent'
The latest launch came as a US envoy made a new appeal for talks with Pyongyang.
"We will seek diplomacy with the DPRK to make tangible progress that increases the security of the United States and our allies," said Sung Kim, the US special representative on North Korea, following talks with his South Korean counterpart in Washington.
"We harbour no hostile intent toward the DPRK and we are hopeful to meeting with them without conditions," Kim told reporters.
But he added that the allies had "a responsibility to implement UN Security Council resolutions", referring to sanctions that North Korea seeks to see lifted.
South Korean President Moon Jae-in is pressing for a formal declaration that the Korean War is over -- hostilities ceased in 1953 with an armistice rather than a peace treaty -- before his term ends next year.
Kim met three times with former president Donald Trump, who boasted of stopping a war but failed to reach a comprehensive agreement on ending North Korea's nuclear programme.
The talks process has been largely at a standstill since a second meeting in Hanoi the following year collapsed over sanctions relief and what Pyongyang would be willing to give up in return.
The Biden administration has said it is willing to meet North Korean officials at any time or place, without preconditions, in its efforts to seek denuclearisation.
Pyongyang is under multiple international sanctions over its banned nuclear weapons and ballistic missile programmes, which it says it needs to protect itself against a US invasion.
In 2017, it tested missiles that can reach the whole of the continental United States and carried out its most powerful nuclear explosion to date. North Korea fires ballistic missile: South's military.