North Korea fired an unidentified ballistic missile from an area near Pukchang, according to the South Korean military's Joint Chiefs of Staff.
The missile flew east about 500 km (over 300 miles), South Korean officials said.
"Our military is closely monitoring North Korea and is maintaining readiness posture," the statement said.
It seems to have fallen into the Sea of Japan, also known as the East Sea, but not within Japan's exclusive economic zone, Japanese Chief Cabinet Secretary Yoshihide Suga said in a press briefing.
Authorities have not received any reports of damage on ships or planes, Suga said.
A White House official said the projectile was a medium-range ballistic missile, a system that North Korea tested in February.
The missile tested in February was a Pukguksong-2, a medium long-range ballistic missile, officials said. It was the first test by Pyongyang since President Donald Trump was inaugurated.
