The rockets were fired off the North's east coast and flew about 70 kilometres over the Sea of Japan (East Sea), a ministry spokesman said.
The ministry said 10 rockets were fired at 6:20 pm (0920 GMT), and eight more at 8:03 pm. Another seven followed, the last at 9:30 pm.
Seoul's military "has maintained a close watch for possible North Korean provocations", the spokesman said.
Yonhap news agency reported that 25 short-range missiles were fired.
The activity coincides with annual South Korea-US military exercises that started in February and will run until mid-April.
Pyongyang routinely condemns such war games as rehearsals for an invasion, while Seoul and Washington say they are purely defensive.
"We urge North Korea to stop provocative activities that will heighten military tension across the border," the spokesman said.
The North has defended its missile and rocket tests as justifiable self-defence drills.
South Korea has called the launches a "reckless provocation" while Washington urged Pyongyang to halt the tests immediately, saying they risked inflaming regional tensions.
Analysts said the comments could indicate the regime is preparing to carry out a fourth atomic test.
"Additional measures will be taken to demonstrate its might one after another as long as the US nuclear threat and blackmail persist as now", the commission said in a statement.
"The US had better roll back its worn-out hostile policy towards (North Korea) as soon as possible and shape a new realistic policy before it is too late."
North Korea and its main ally China want a resumption of six-party talks on the North's nuclear weapons programme. But the United States and South Korea both insist that Pyongyang first demonstrates some tangible commitment to abandoning nuclear weapons.
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