The Kyushu Electric Power Company told CNN that it restarted the Dai-ni reactor at Sendai Nuclear Power Plant Thursday morning, as planned and the reactor is scheduled to be fully operational and producing power by October 21.
The move comes despite widespread opposition to the use of nuclear energy in Japan.
The plant, in the country's southern Kagoshima prefecture, on the southern island of Kyushu, also hosts the only other reactor to have been restarted since the disaster, which saw much of Fukushima prefecture, north of the capital, Tokyo, evacuated in 2011.
The company restarted the Dai-ichi -- number one -- reactor at the same Sendai power plant on August 12, 2015.
The Fukushima disaster, which occurred when a tsunami caused by a massive earthquake off the coast of Japan, flooded the plant's reactors, causing a meltdown at three of them.
It led to the mothballing of Japan's nuclear energy production by May 2012.
The decision to restart the country's nuclear energy production in the wake of the disaster has been met with widespread opposition, despite what Prime Minister Shinzo Abe calls the "world's toughest" nuclear power safety measures.
Abe's government, also under fire for other unpopular decisions including the redrafting of the country's constitution to allow military intervention overseas, hopes to use nuclear power to provide 20 to 22% of Japan's total electricity needs, Japanese media reports.