The South Korean Joint Chiefs of Staff (JCS) said it detected the launch from the North's western port city of Nampo at 6:20 p.m. It did not elaborate further, Yonhap news agency reported.
"While strengthening its monitoring and vigilance, our military is maintaining a full readiness posture in close cooperation with the United States," the JCS said in a text message sent to reporters.
The latest launch came as the allies are preparing to kick off the Freedom Shield exercise set to take place from Monday through March 23. The exercise is to proceed concurrently with the large-scale field training exercise, called the Warrior Shield.
Last month, Pyongyang warned Seoul and Washington would face "unprecedentedly" strong counteractions should they press ahead with this year's plans for combined drills, which it has decried as preparations for a war of aggression.