It says the missile was launched in waters east of the Korean peninsula - the latest in a recent series of tests.
North Korea, which is developing nuclear weapons, is banned by UN resolutions from any use of ballistic missile technology.
The US and South Korea on Friday agreed to deploy a missile defence system to counter threats from Pyongyang.
The South Korean military says the missile was launched at about 11:30 local time (02:30 GMT) off North Korea's port of Sinpo.
