"From 21 April, North Korea will stop nuclear tests and launches of intercontinental ballistic missiles," the country's state news agency said.
Kim said further tests were unnecessary because Pyongyang's nuclear capabilities had been "verified".
The surprise announcement comes as North Korea prepares for historic talks with South Korea and the US.
Kim is due to meet his South Korean counterpart Moon Jae-in next week for the first inter-Korean summit in over a decade, and US President Donald Trump by June.
Both countries have been pushing Pyongyang to denuclearize and they reacted positively to the latest development.
"This is very good news for North Korea and the World - big progress!" Trump tweeted after the announcement.