The move, announced on Tuesday, will put into motion a plan approved by the Japanese government two years ago, as crucial to decommissioning the plant operated by Tokyo Electric Power Company (Tepco).
But it has drawn strong criticism from China and other neighbouring countries, and also local fishing groups who fear reputational damage and a threat to their livelihood.
Japanese Prime Minister Fumio Kishida said on Tuesday morning: “I have asked Tepco to swiftly prepare for the water discharge in accordance with the plan approved by the Nuclear Regulation Authority, and expect the water release to start on August 24, weather conditions permitting.”
The announcement comes a day after the government said it had won “a degree of understanding” from the fishing industry over the release of the water, even as a fishing group said it still feared the reputational damage would ruin livelihoods.
“I promise that we will take on the entire responsibility of ensuring the fishing industry can continue to make their living, even if that will take decades,” Mr Kishida said on Monday.
Japan has said the water release is safe. The International Atomic Energy Agency (IAEA), the UN nuclear watchdog, greenlighted the plan in July, saying that it met international standards and that the impact it would have on people and the environment was “negligible”.
The water was used to cool the fuel rods of Fukushima Daiichi Nuclear Power Plant, after it melted down in an accident caused by a huge tsunami that battered Japan’s eastern coast in 2011.
Tanks on the site now hold about 1.3 million tonnes of radioactive water - enough to fill 500 Olympic-sized swimming pools - which have been distilled.
Japan says the water will be filtered to remove most radioactive elements except for tritium - an isotope of hydrogen that is difficult to separate from water. The treated water will be diluted to well below internationally approved levels of tritium before being released into the Pacific.
Yet some neighbouring countries have expressed scepticism over the safety of the plan, with China the biggest critic.
Foreign ministry spokesman Wang Wenbin said in July that Japan had shown selfishness and arrogance, and had not fully consulted the international community about the water release.
China bans seafood imports from 10 prefectures in Japan, including Fukushima and the capital, Tokyo. Seafood imports from other prefectures are allowed but must pass radioactivity tests and have proof they were produced outside the 10 banned prefectures.
South Korean activists have also protested the plan, although Seoul has concluded from its own study that the water release meets international standards and said it respects the IAEA’s assessment.
Pacific Island nations have been split over the matter, given their own history of being nuclear testing sites for the United States and France. Fiji’s prime minister, Sitiveni Rabuka, issued a statement on Monday saying that he backed the IAEA report, but acknowledged that the issue is controversial in the Pacific Islands.
Mr Kishida said on Tuesday that he believed an “accurate understanding” of the matter was spreading in the international community.
The process of pumping more than a million tonnes of treated water from the power plant is expected to take decades.
A Japanese official said the first test results of the seawater after the discharge may be available at the start of September. Japan will also test fish in the waters near the plant, and make the test results available on the agriculture ministry’s website.