The International Atomic Energy Agency (IAEA) has endorsed the release, which it says is similar to the disposal of waste water at nuclear plants elsewhere in the world.
The release is unlikely to begin for at least two years but has already sparked opposition from local fishing communities and concern in Beijing and Seoul.
Chinese foreign ministry spokesman Zhao Lijian urged Japan to "act in a responsible manner".
It includes water used to cool the plant, as well as rain and groundwater that seeps in daily.
Cabinet ministers endorsed the release as the best option for handling the massive amount of water that has been stored at the Fukushima Daiichi nuclear power plant since the massive quake and tsunami in 2011 caused reactor meltdowns and leaks of cooling water from the damaged reactors.
"The decision ... was a unilateral move made without enough discussion or understanding from us, South Korea, which is the closest country geographically", Koo said at a press briefing, adding that South Koreans including lawmakers and civic groups were also strongly opposed to it.
The government plans to decide on Tuesday at the earliest to release the treated water into the ocean, after diluting it to levels far below national regulations.
Tomoaki Kobayakawa, president of plant operator TEPCO, said it would "take thorough measures to prevent bad rumours" affecting local industries.
South Korea expressed "serious concerns that the decision could bring a direct and indirect impact on the safety of our people and surrounding environment".
"This approach is extremely irresponsible and will seriously damage global public health and safety", it said.
"In this unique and challenging situation, Japan has weighed the options and effects, has been transparent about its decision, and appears to have adopted an approach in accordance with globally accepted nuclear safety standards".
Wastewater produced by the crippled nuclear plant is stored in tanks in the compound that are set to fill up sometime next year.
Japanese Prime Minister Yoshihide Suga said on Tuesday that his government has chose to discharge contaminated radioactive wastewater in Fukushima Prefecture into the sea amid domestic and worldwide opposition. It's not something new. However, things like tritium, a radioactive byproduct of nuclear reactors, are hard to filter out.
Anti-nuclear activist group Greenpeace slammed Japan's government for having "once again failed the people of Fukushima".
"To safeguard worldwide public interests and Chinese people's health and safety, China has expressed grave concern to the Japanese side through the diplomatic channel", Zhao said Monday.
Experts say the element is only harmful to humans in large doses and with dilution the treated water poses no scientifically detectable risk.