The North fired the missile between 11 a.m. and noon, local time, into the ocean, Yonhap News Agency reported. The country fired three missiles into the same location Saturday and a fourth Sunday.
"North Korea again launched a short-range projectile that appears to be a KN-02 [surface-to-surface] missile," a defense ministry official said. "We are closely watching the movements of the North's military in case of further launches."
South Korea condemned the launches Sunday, calling the action provocative and "deplorable."
"We are currently trying to determine what the projectile was and the North's intentions [behind the launch]," a military official said. "It appears that the North is trying to renew military tensions on the Korean Peninsula."
The missiles appeared to be launched from a mobile launcher and flew approximately 75 miles.
"Whether it's a test-firing or armed demonstration, North Korea should not engage in tension-creating acts," said Kim Jang-soo, head of the national security office.
North Korea said the launches were normal military exercises.
"Conducting military drills to build up a strong deterrence capability is a legitimate right of any sovereign country," said the secretariat of the Committee for the Peaceful Reunification of Korea.