The test is the fourth reported since the beginning of March
North Korea has tested a number of short-range missiles - the latest in a series of launches condemned by other nations.
The projectiles fired on Monday flew for 200km and landed in waters east of North Korea, South Korea's Joint Chiefs of Staff said in a statement.
South Korean news agency Yonhap, citing an unidentified government source, said the South Korean military was trying to find what types of missile and projectile were fired.
The test is the fourth reported in North Korea since the beginning of March, as the secretive state steps up its defiance of US and UN sanctions imposed in response to recent launches.
It came three days after the South said the North fired a medium-range missile into the sea for the first time since early 2014.
North Korea has condemned ongoing annual South Korean-US military drills that it sees as an invasion rehearsal.
The secretive state has threatened to launch pre-emptive strikes against Seoul and the US in recent weeks - warning its nuclear missiles could burn Manhattan "to ashes".
Its leader Kim Jong-Un said last week that the country would soon test a nuclear warhead and ballistic missiles capable of carrying nuclear warheads in what would be a direct violation of UN resolutions that have the backing of Pyongyang's chief ally, China.