The Japanese government announced on Tuesday at 10:46 p.m., local time, that North Korea launched at least one missile.
It likely flew over Japan's southwestern prefecture of Okinawa toward the Pacific Ocean.
The government issued an evacuation alert, which was later lifted. The Self-Defense Forces took no steps to destroy the projectile.
The government advises the public to stay away from anything that could be debris. Any sightings of possible debris should be reported to authorities.
North Korea had warned of plans to launch what it calls an artificial satellite into orbit sometime between November 22 and 30.
In an extremely rare occurrence, the latest launch came more than one hour ahead of that timeframe.
South Korea's military says the projectile is what Pyongyang calls a military reconnaissance satellite.
The latest launch is North Korea's third attempt at putting a so-called satellite into orbit, after failures in May and August.