North Korea has fired "several projectiles" into sea off its eastern coast, the South Korean military says.
The missile was launched from a site in the North Korean province of Gangwon and flew for about 250km (150 miles), officials in the South said.
North Korea has accelerated its missile testing programme.
Since firing an intercontinental ballistic weapon last month, Pyongyang has threatened to aim missiles at the US Pacific territory of Guam.
The projectiles were launched at 06:49 on Saturday (21:49 GMT Friday), South Korea's Defence Ministry said.
"The military is keeping a tight surveillance over the North to cope with further provocations", the ministry said.
Thousands of US and South Korean troops are currently taking part in joint military drills, which are mainly largely computer-simulated exercises.