The missiles were fired from areas around Hwangju country, in North Hwanghae province, towards the Sea of Japan, the Joint Chiefs said in a statement reported by CNN.
This comes just under two weeks after Pyongyang test-fired a submarine-launched ballistic missile. That missile entered Japan's air defense identification zone, the first time that has happened.
It also comes at a potentially embarrassing time for North Korea's only real ally, China, which is currently hosting the Group of Nations summit in Hangzhou.
At the G20, Chinese President Xi Jinping told his South Korean counterpart on Monday that Beijing opposes the deployment of the United States' THAAD missile defense system to South Korea.
"Mishandling the issue is not conducive to strategic stability in the region and could intensify disputes," Xi told Park Geun-hye.
The stated purpose of the THAAD system is to defend South Korea from missile attack from the North.
South Korea announced the deployment of the missile system in July but it's not yet in operation.