The Pentagon said it was assessing the “probable” launch, which happened at about 03:30 local time (18:30 GMT).
South Korean news agency Yonhap reported that the missile flew eastward from Pyongsong, South Pyongan province.
The North has carried out a series of missile tests in recent months, raising tensions with its neighbours and with the US.
It is not clear how far the latest missile went or whether it flew over Japan as others did earlier this year.
North Korea’s last ballistic missile test was in September, days after it had conducted its sixth nuclear test.
US President Donald Trump was briefed while the missile was still in the air, the White House has said. He was visiting Congress at the time, but has since returned to the White House.
South Korea’s military said it had responded to the launch with a missile exercise of its own.
North Korea is thought to be focusing efforts on building long-range missiles with the potential of reaching the mainland continental US.
Pyongyang officials said the first of the longer-range missiles it tested in July could hit “any part of the world”, but the US military called it an intermediate-range missile instead.
Its last nuclear test reportedly involved a miniaturised hydrogen bomb that could be loaded on to a long-range missile.