Kyushu Electric Power Company told CNN Tuesday that it had reactivated No. 1 reactor
at its Sendai nuclear power plant in Kagoshima Prefecture, on the southern island of Kyushu.
The plant's second reactor could be restarted in October, it added.
All of Japan's 48 nuclear reactors were gradually taken offline when a tsunami
triggered by a massive earthquake sent a wall of water crashing into the Fukushima-Daiichi nuclear plant four years ago, causing a series of meltdowns.
Since then, the island nation has imported greater amounts of expensive natural gas and coal to meet its energy needs.
Japanese Prime Minister Shinzo Abe has pushed for a return to nuclear energy, arguing it is essential to the country's economic recovery to reduce the skyrocketing utility bills associated with energy imports.
Prior to the Fukushima disaster, about 30% of Japan's energy was nuclear generated.
But opinion polls have consistently shown public opposition to a nuclear restart.
Japan's nuclear watchdog, the Nuclear Regulation Authority, had previously given a green light to the reopening of reactors 3 and 4 of the Kansai Electric Power Company's Takahama nuclear plant.
But locals successfully petitioned the court in Fukui Prefecture, where the plant is located, to block the move, raising concerns about whether the reactors would survive a strong earthquake.
Seismic and volcanic risks
And anti-nuclear campaigners on Tuesday voiced similar concerns about Sendai.
"The Nuclear Regulation Authority and Kyushu have not applied a robust enough risk assessment -- they have disregarded the seismic and volcanic risks involved, while the reactors at Sendai are also aging," Ai Kashiwagi, of Greenpeace Japan, told CNN.
With other reactors around the country set to follow suit, Kashiwagi warned that Japan should be moving away from nuclear and other dirty fuels. "Renewable energy sources such as solar, wind and thermal have so much potential here," she said.
Chief Cabinet Secretary Yoshihide Suga insisted that the Japanese government is placing priority on safety over anything else. But he told reporters Tuesday that the restart was very important for energy security, the economy, and measures to address global warming.
He said the country's new regulations governing the operation of nuclear plants were the world's toughest.