Beijing said the test was "routine" but analysts say that is odd as the last such test was in 1980.
China has said it successfully launched an intercontinental ballistic missile (ICBM) carrying a dummy warhead into the Pacific Ocean.
The ICMB was launched at 08:44 local time (04:44 GMT) on Wednesday and "fell into expected sea areas", Beijing's defence ministry said, adding that the test launch was "routine" and part of its "annual training".
The type of missile and its flight path remained unclear, but Chinese state media said Beijing had "informed the countries concerned in advance".
Japan later said that it received "no notice" of the test launch.
China's nuclear weapon tests usually take place domestically, and it previously test-fired ICBMs west into the Taklamakan Desert in the Xinjiang region.
This is believed to be the first time since 1980 that it launched an ICBM into international waters.