It was not clear how far the missile travelled, but a US government source told Reuters the test failed.
The missile was fired from a site in South Pyeongan province north of Pyongyang in the early hours of Saturday morning local time, South Korea's military chiefs of staff said.
They said the type of missile was not yet known.
There has been no word from North Korea.
The reported launch occurred a matter of hours after the US Secretary of State Rex Tillerson called on the rest of the world to help force North Korea to give up its nuclear ambitions.
Tensions in the region have increased lately, with both North and South Korea conducting military exercises.
Less than a week ago, a North Korean missile test failed. The US military reported that the missile exploded within seconds of being launched.
A few days later, on Wednesday, the US installed anti-missile equipment at a site in South Korea. The Terminal High Altitude Area Defence System (Thaad) is designed to shoot down short- and medium-range ballistic missiles as they are approaching their target in the final phase of flight.