The fireball left a trail of sparks as it travelled in the city of Niigata in the north of the country in the early hours of Monday morning.
The flying orb appeared in the sky for about 15 seconds before it disappeared.
Some experts believe the bright ball of light, which was filmed by Nippon TV’s weather camera, is actually a meteor.
One astronomer told NBC the fireball could have been caused by space debris, such as satellite fragments burning up as they reentered the atmosphere.
The American Meteor Society told 24 Gio the fireball was “brighter than normal” and its green hue indicated high levels of nickel.
“It has the same brightness as the planet Venus in the sky in the early morning or at night,” it said.
But some UFO hunters remain sceptical, claiming the sighting was in fact, of an alien ship.
Scott C Waring, who runs the UFO Sightings Daily blog, said: “This UFO tried hard to disguise itself as a free slowing meteor over Japan – however, it made some amateurish mistakes.
“Firstly, it’s glowing a colour most meteors don’t glow… green. Secondly meteorites do not fly horizontal while that low to the planet.
“Thirdly – and this was the stupidest mistake the aliens made – the main ship turned off its glow for only one-tenth of a second.
“At that moment a silver disk came out and moved back slowly from the main ship.”
He added: “See, even aliens make mistakes – this pilot – he made a lot.”