"North Korea launched a projectile which was believed to be a submarine-launched ballistic missile (SLBM) around 6:30 pm (0930 GMT) in the East Sea (Sea of Japan)" near the northeastern port of Sinpo", a defence ministry spokesman said.
"We are keeping close tabs on the North Korean military and maintaining a full defence posture", he said.
It was not immediately known whether the launch was a success, he added.
North Korea has been pushing to acquire SLBM capability that would take its nuclear strike threat to a new level, allowing deployment far beyond the Korean peninsula and the potential to retaliate in the event of a nuclear attack.