The launches on Thursday also followed leader Kim Jong-un's surprise weekend visit to China, the North's main ally and economic pipeline, to push for greater co-operation.
South Korea's Joint Chiefs of Staff said it detected about 10 short-range ballistic missiles fired from the North's eastern costal town of Wonsan.
It said South Korea's military bolstered its surveillance posture and was closely exchanging information about the launches with the US and Japan.
Animosity between the Koreas have deepened after September's landmine blasts that injured three South Korean soldiers.
Seoul accused North Korea of planting mines south of the military demarcation line that caused the September 21 blasts, but Pyongyang has denied responsibility for the explosions and warned of retaliation if provoked.
Last week, North Korea said it would restart propaganda broadcasts and deploy new weapons systems along the border in protest of what it called repeated warning shots by South Korean troops at border areas.
South Korea later proposed talks on clarifying the border to prevent accidental clashes, but North Korea has not responded.
Thursday's launches were the latest in a series of weapons tests by North Korea in recent weeks, which involved submarine-launched cruise missiles and hypersonic missiles.
Experts say North Korea likely aimed to pressure the US into making bigger concessions as President Donald Trump expressed his desire to restore diplomacy with Kim.
On Saturday, Kim visited the Chinese border city of Dandong via a newly inaugurated bridge to pay respect at a memorial marking China's support of North Korea during the 1950-53 Korean War.
It was his sixth visit to China since taking office in late 2011.
At the memorial, Kim said North Korea will firmly carry forward "the invincible" friendship with China.
Later he met top Chinese Communist Party official Wang Huning and reaffirmed his resolve to develop bilateral co-operation.
Kim has recently taken steps to bolster ties with China after years of prioritising ties with Russia by sending troops and ammunitions to support its war against Ukraine.
