There were no immediate reports of damage on land.
The quake hit at 2:10am on Saturday, Tokyo time (5:10pm GMT) about 290 kilometers (170 miles) off Fukushima. Japanese broadcaster NHK reported that Tokyo Electric Power Co., the operator of the Fukushima plant, ordered workers near the coast to move to higher ground.
The tremor was felt in Tokyo, some 300 miles (480 kilometers) away.
All but two of Japan's 50 reactors have been offline since the March 2011 magnitude-9.0 earthquake and ensuing tsunami triggered multiple meltdowns and massive radiation leaks at the Fukushima Dai-ichi nuclear power plant, about 250 kilometers (160 miles) northeast of Tokyo. About 19,000 people were killed.