Fukushima Daiichi nuclear power plant in Okuma town, Fukushima prefecture, north of Tokyo, Tuesday, April 13, 2021.
An extensive pumping and filtration system known as "ALPS (Advanced Liquid Processing System)" extracts tonnes of newly contaminated water each day and filters out most radioactive elements.
He said the release would happen only "after ensuring the safety levels of the water" and alongside measures to "prevent reputational damage".
TEPCO and government officials say tritium, which is not harmful in small amounts, cannot be removed from the water, but all other selected radionuclides can be reduced to levels allowed for release.
But local fishing communities fear releasing the water will undermine years of work to restore confidence in seafood from the region.
Ban also said releasing the water into the sea is likely to damage the reputations of the local fishery and tourism industries, and hamper recovery from the nuclear accident.
South Korea expressed "serious concerns that the decision could bring a direct and indirect impact on the safety of our people and surrounding environment".
China's foreign ministry on Tuesday criticised the Japanese government's decision, calling it "extremely irresponsible".
"The United States is aware that the GOJ examined several options related to the management of the treated water now being stored onsite at the Fukushima Daiichi site", the US Department of State said in a press release Tuesday, referring to the government of Japan.
"This approach is extremely irresponsible and will seriously damage global public health and safety", it said.
Japanese Prime Minister Yoshihide Suga on Monday said dumping treated water containing radioactive substances to the ocean is "unavoidable" and there is "no time to delay" for the reconstruction of Fukushima, Global Times reported. The treated water, stored in tanks, has been building up, with storage capacity expected to run out as early as fall next year.
"Releasing into the ocean is done elsewhere. There is no scandal here", IAEA director general Rafael Mariano Grossi said past year.
Local activist groups asked the Korean government to seriously consider its legal options in response to the "unjustified decision", saying the process would harm the interests of Korea and its people.
China said it has conveyed its "serious concern" to Japan, calling on Prime Minister Yoshihide Suga's government to make a cautious decision to protect the public interest of worldwide society as well as the health and safety of Chinese citizens.
Government spokesman Katsonobu Kato said Tuesday that the water would be diluted to contain tritium at levels far below either domestic or World Health Organization standards, with the IAEA monitoring the process.
"Releasing the water into the ocean will return to haunt us", they said.