Japan also reported the launch, with chief cabinet secretary Hirokazu Matsuno condemning it as a threat to peace and security.
In less than two weeks, nuclear-armed North Korea has conducted three other missile tests, an unusually rapid series of launches.
It said two of them involved single "hypersonic missiles" capable of high speed and manoeuvring after launch, while a test on Friday involved a pair of short-range ballistic missiles (SRBMs) fired from train cars.
Monday's launch appeared to involve two SRBMs fired east from Sunan Airfield in Pyongyang, South Korea's Joint Chiefs of Staff (JCS) said in a statement.
The missiles fired on Monday travelled about 380km to a maximum altitude of 42km, the JCS said in a statement.
Japanese Defence Minister Nobuo Kishi said the missiles appeared to have landed in the ocean near North Korea's east coast.
"The repeated launching of North Korea's ballistic missiles is a grave problem for the international community, including Japan," Kishi added, noting that the launches were a violation of UN Security Council resolutions that ban North Korea from all ballistic missile development.
The US military's Indo-Pacific Command said "these missile launches highlight the destabilising impact of (North Korea's ) illicit weapons program".
North Korea has not tested its longest-range intercontinental ballistic missiles (ICBMs) or nuclear weapons since 2017, but after denuclearisation talks stalled in 2019, it began unveiling and testing a range of new SRBM designs.
Many of the latest SRBMs, including the hypersonic missiles, appear designed to evade missile defences. North Korea has also vowed to pursue tactical nuclear weapons, which could allow it to deploy nuclear warheads on SRBMs.
The latest launches have drawn both condemnation and an appeal for dialogue from a US administration that has imposed new sanctions over North Korean missile launches and is pushing for more.
US President Joe Biden's administration imposed its first new sanctions on Pyongyang on Wednesday, and called on the UN Security Council to blacklist several North Korean individuals and entities.
It also repeated calls for North Korea to return to talks aimed at reducing tension and persuading it to surrender its arsenal of nuclear weapons and ballistic missiles.
North Korea has defended the missile tests as its sovereign right to self-defence and accused the US of intentionally intensifying confrontation with new sanctions.
South Korea's national security council held an emergency meeting after Monday's test, with members stressing that "above all else, it is essential to start dialogue as soon as possible in order for the situation on the Korean Peninsula to not become more strained and to restore stability", the presidential Blue House said in a statement.
Zhao Tong, a Beijing-based nuclear policy expert at the Carnegie Endowment for International Peace, said North Korea had few reasons to hold back its missile development.
Leader Kim appeared to have little hope of a breakthrough with the United States, and China's sympathy for North Korea and antipathy towards the United States could encourage North Korea to think that China was unlikely to support any effort by the international community to censure it for the tests, he added.
"North Korea may think this is a safe time to advance its missile development," Zhao said.
