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 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.
North Korea used the airport to test fire the Hwasong-12 intermediate-range ballistic missile (IRBM) in 2017, with leader Kim Jong Un in attendance.
The missiles fired on Monday travelled about 380 km (236 miles) to a maximum altitude of 42 km (26 miles), the JCS said in a statement.