Japanese meteorologists have reported that a tsunami of at least two metres (6.5 feet) high could hit Hokkaido and Honshu islands after 2110 (1210 GMT).
Warnings have been broadcast on all Japanese TV and radio stations advising people to move to higher ground.
The alert has been raised following the earthquake this morning of about 8.1 magnitude, which struck the Kuril Islands, north of Japan.
The earthquake struck about 390km (240 miles) east of Iturup, known as Etorofu in Japan, at 1115 GMT, the Japanese meteorological agency explained.
The first waves of the tsunami were expected to arrive shortly after noon British time. The site of the quake was roughly 1,700 km northeast of Tokyo, Reuters has confirmed.
Hokkaido residents, however, reported feeling little to no shaking from the earthquake, Reuters has said.
