South Korea says the missiles flew cross-country after being fired from a western inland area south of the North Korean capital of Pyongyang.
North Korea has fired two short-range ballistic missiles, South Korea's military said, the latest in its flurry of weapons tests in recent weeks.
Monday's launch comes as Seoul and Washington are carrying out a joint amphibious landing exercise, and just days after they wrapped up their largest combined military drills in five years.
North Korea views all such exercises as rehearsals for invasion and has repeatedly warned it would take "overwhelming" action in response.
"Our military detected two short-range ballistic missiles fired from around Junghwa area in North Hwanghae province from 07:47 am (2247 GMT) towards the East Sea," South Korea's Joint Chiefs of Staff (JCS) said, referring to the body of water also known as the Sea of Japan.
"Our military has strengthened surveillance and vigilance against additional launches, while maintaining a full readiness posture through close cooperation between South Korea and the United States," it added.
Japan's defence ministry also confirmed the launch, with Japanese media citing officials as saying both missiles are believed to have flown on an irregular trajectory before falling outside of Japan's exclusive economic zone.
READ MORE: North Korea tests new underwater 'nuclear attack drone'
'Stronger war deterrents'
On Friday, North Korea's state media claimed the joint training between Seoul and Washington was practice for "occupying" North Korea, which called for "stronger war deterrents" including "the more developed, multi-faceted and offensive nuclear attack capability".
Pyongyang has carried out military drills of its own in response, including test-firing a new nuclear-capable underwater drone and conducting its second intercontinental ballistic missile launch this year.
Analysts previously said North Korea would likely use the exercises as an excuse to carry out more missile launches and perhaps even a nuclear test.
North Korea's state media said Friday the "underwater nuclear attack drone" drill, which was personally overseen by leader Kim Jong Un, was staged "to alert the enemy to an actual nuclear crisis".
The weapon's mission is to "stealthily infiltrate into operational waters and make a super-scale radioactive tsunami ... to destroy naval striker groups and major operational ports of the enemy," the Korean Central News Agency reported.
The agency also said Pyongyang had fired strategic cruise missiles "tipped with a test warhead simulating a nuclear warhead" on Wednesday.
North Korea last year declared itself an "irreversible" nuclear power and Kim recently called for an "exponential" increase in weapons production, including tactical nuclear weapons.
Kim also ordered the North Korean military this month to intensify drills to prepare for a "real war".
READ MORE: Kim Jong-un supervises North Korea troops simulating attack on South