Little is known about the projectiles, except that they were launched from the North's east coast towards the Sea of Japan, also known as the East Sea.
Last May saw the first missile tests after an 18-month freeze. As the year progressed, many more followed.
North Korea has historically stepped up missile testing in the spring.
"The military is monitoring for additional launches and maintaining readiness," said South Korea's Joint Chiefs of Staff in a statement.
The tests come just days after South Korea and the US announced they were postponing the annual joint
drills that anger the North, amid concern over the coronavirus.
At the start of the year, North Korean leader Kim Jong-un said he was ending the suspension of nuclear and long-range missile tests, as talks between the US and North Korea ground to a halt.
The North Korean leader threatened that the world would "witness a new strategic weapon... in the near future".
US President Donald Trump last met Kim Jong-un at the inter-Korean border in June 2019.
North Korea conducted several weapons tests in 2019, in what was seen as an attempt to pressure the US into concessions.