The ship-to-ship missile was launched from a North Korean vessel in the Sea of Japan on July 6 and flew less than 100 kilometers (62 miles), the Yonhap news agency reported, citing South Korean military officials.
The exact type of the projectile is unknown, but this missile had reportedly been tested by Pyongyang before 6 July.
"We see this as part of the North's regular maritime exercise ... We are closely monitoring the North's military moves and maintain a tight readiness posture," Joint Chiefs of Staff spokesperson Col. Kim Jun-rak said, as quoted by Yonhap.
For the first time, North Korea tested a self-guided anti-ship missile in February 2015. The previous missile test was conducted by Pyongyang on 14 April when several short-range surface-to-ship cruise missiles were fired into the Sea of Japan.