The launches took place on Tuesday from the country's South Hwanghae Province at around 7:40 a.m. (2240 GMT on Monday), South Korea's Joint Chiefs of Staff (JCS) said.
The South Korean military was on high alert and maintained full readiness posture under close coordination with the United States, the JCS alleged in a statement.
Japanese Prime Minister Fumio Kishida also said his country was collecting information on the missiles, adding that Tokyo had not confirmed any damage within the country related to the launches.
The launches came a day after South Korea and the US began 11 days of joint drills, dubbed "Freedom Shield 23," which are being held on a scale not seen since 2017. The drills feature field exercises, including amphibious landings.
Right ahead of the exercises, the North test-fired two "strategic" cruise missiles from a submarine. The North's official Korean Central News Agency (KCNA) said the launch had taken place from waters off the eastern coastal city of Sinpo, with the projectiles flying some 1,500 kilometers (932 miles) before hitting an underwater target.
On Sunday, the KCNA reported the country has decided to take "important practical" war deterrence measures, saying, "War provocations of the US and South Korea are reaching the red line."
The US and the South have held several joint drills since the beginning of the year. Pyongyang views the war games as potential rehearsals for the invasion of its territory.
In a rare move, the South Korean military this month revealed that special forces belonging to Seoul and Washington were staging "Teak Knife" military exercises, which involved simulating precision strikes on key facilities in North Korea.
On Thursday, the country's leader Kim Jong Un, who was overseeing a short-range ballistic missile launching exercise, ordered his military to intensify drills to prepare for a "real war."
Last year, the North declared itself an "irreversible" nuclear power and conducted a record number of missile launches and drills in an effort to boost its nuclear deterrence and make more weapons fully operational.
MNA/PR