It was the North's third ballistic missile test in as many weeks and the 12th this year – carried out in defiance of UN sanctions warnings and US threats of possible military action.
US military monitors said the short-range missile flew for six minutes, while Japan said it fell into its exclusive economic zone, extending 200 nautical miles from the coast.
The launch went ahead despite tough talk from US President Donald Trump, who promised last week at the G7 summit that the "big problem" of North Korea "will be solved".
Japan's Prime Minister Shinzo Abe swiftly condemned the test and vowed concerted action with its US ally.
"We will never tolerate North Korea's continued provocations that ignore repeated warnings by the international community," Abe told reporters.
"As agreed during the G7 summit, the North Korean problem is the international community's top priority. In order to deter North Korea, we will take concrete action with the United States."
The North has been stepping up efforts towards its ultimate goal -- developing an intercontinental ballistic missile (ICBM) that can deliver a nuclear warhead to the continental US.
Monday's test also marked the second time this year that a North Korean missile fell provocatively close to its neighbour Japan. South Korea's military said the Scud-type missile travelled for 450 km (280 miles).
Trump says North Korea shows "great disrespect" to China with missile
US President Donald Trump said North Korea showed disrespect to its major ally China after it fired a short-range ballistic missile on Monday that landed in the sea off its east coast.
"North Korea has shown great disrespect for their neighbor, China, by shooting off yet another ballistic missile ... but China is trying hard!" Trump said in a post on Twitter.
Conflict 'catastrophic'
Despite Trump's strident warnings, Secretary of Defense James Mattis said in an interview which aired Sunday before the launch that a war with North Korea would be "catastrophic".
"The North Korean regime has hundreds of artillery cannons and rocket launchers within range of one of the most densely populated cities on Earth, which is the capital of South Korea," he told CBS News.
"This regime is a threat to the region, to Japan, to South Korea. And in the event of war, they would bring danger to China and to Russia as well.
"But the bottom line is, it would be a catastrophic war if this turns into a combat, if we're not able to resolve this situation through diplomatic means."
Bishop condemns North Korea's missile testing
Mattis declined to say what kind of action from Pyongyang would constitute a "red line" for Washington, saying the administration needs "political manoeuvre room."
The latest launch demonstrates the North's determination to secure more leverage in any future negotiations with the US, said Cho Han-Bum, analyst at the Korea Institute for National Unification.
"The North, despite its series of provocations, has not crossed the ultimate red line, which would be staging another nuclear test or a successful ICBM test," Cho said.
"Today's launch is the North's way of saying to the world, 'It wouldn't be easy to make us suspend our weapons programmes even if you manage to pressure me into negotiations'," he said.
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'Direct challenge'
South Korea's new president Moon Jae-In ordered a meeting of the national security council to assess the launch, which came a day after North Korea said its leader Kim Jong-Un had overseen a test of a new anti-aircraft weapons system.
The South condemned the missile test as a "grave threat" and a challenge to Moon, who advocates dialogue with the North in a break from his conservative predecessors.
"That the North repeated such provocations after the inauguration of our new leadership... is a direct challenge to our demand for peace and denuclearisation of the Korean peninsula," the foreign ministry said.
The missile launches, and Pyongyang's threat to stage its sixth nuclear test, have prompted calls for tougher UN sanctions and a warning from Trump that military intervention was an option under consideration.
Following North Korea's test-firing earlier this month of what analysts said was its longest-range rocket yet, the UN Security Council vowed to push all countries to tighten sanctions against Pyongyang.
But China, the North's main trade partner and ally, has made it clear that the push for talks - and not more sanctions - is the priority.
The US has said it is willing to enter into talks with North Korea, but only if it halts its missile and nuclear tests.
