The tremor at 4.27pm on the island of Kyushu also measured at the highest possible level seven on Japan’s own Shindo scale of shaking, the Japan Meteorological Agency said.
The tsunami could measure up to one metre, the agency said.
Broadcaster NHK said that waves of up to that level may already have been recorded.
No immediate abnormalities were recorded at nuclear power plants, the Nuclear Regulation Authority said.
About 45,000 households and facilities have no power in the Kumamoto region, according to local utility Kyushu Electric Power.
The firm added that three nuclear reactors operating at the time in the region were continuing their normal operations.
Around half an hour after the quake, a local NHK staffer wearing a helmet reported live on air that the shaking was continuing, with the newsroom shaking so violently it was difficult to remain standing.
“Shaking is continuing as I speak. Including myself, there may be some people who feel their hearts racing,” he said.
“When the quake struck, the shaking was so big that I could not stand up. Please be aware of possibility that shaking might continue after this.”
In 2016, Kumamoto was hit two devastating earthquakes — one of magnitude 6.5, followed two days later by one of magnitude 7.3 — which killed 273 people and injured more than 2,800 others.
A quake with a magnitude of 7.2 hit northern Japan on June 25 but caused no deaths or major damage.
Japan is one of the world’s most seismically active countries, sitting on top of four major tectonic plates along the western edge of the Pacific “Ring of Fire”.
The archipelago, home to around 125 million people, typically experiences hundreds of jolts every year and accounts for about 18 percent of the world’s earthquakes.
The vast majority are mild, although the damage they cause varies according to their location and the depth below the Earth’s surface at which they strike.
It is haunted by the memory of a massive 9.0-magnitude undersea quake in 2011, which triggered a tsunami that killed or left missing around 18,500 people and wrecked the Fukushima nuclear plant.
On April 20 this year, a tremor measuring 7.7 hit the country’s north, injuring at least 10 people and shaking large buildings in Tokyo.
This prompted authorities to issue a special advisory warning of an increased risk of earthquakes of magnitude 8.0 or stronger.
The advisory was lifted after a week. (AFP)
