Tokyo says work to release water used to cool nuclear fuel will begin in about two years.
The final approval comes after years of debate and is expected to take decades to complete.
The Asian country says water will be treated and diluted so radiation levels are below those set for drinking water.
The move has been strongly opposed by the local fishing industry, China and South Korea.
Chinese foreign ministry spokesman Zhao Lijian also urged Japan to “act in a responsible manner” so as:
To safeguard international public interests and Chinese people’s health and safety, China has expressed grave concern to the Japanese side through the diplomatic channel.
The US appears to support Japan’s decision, however, saying it seemed to have “adopted an approach in accordance with globally accepted nuclear safety standards”.
Environmental groups like Greenpeace have long expressed their opposition to releasing the water into the ocean saying the plans showed the Japanese government “once again failed the people of Fukushima”.
Currently, the radioactive water is treated in a complex filtration process that removes most of the radioactive elements, but some remain, including tritium, deemed harmful to humans only in very large doses.
According to a Reuters report, about 1.3 million tonnes of radioactive water, or enough to fill 500 Olympic-sized swimming pools, are currently stored in these tanks.
Japan argues that the release of the wastewater is safe as it is processed to remove almost all radioactive elements and will be greatly diluted.
The plan has the backing of the International Atomic Energy Agency, which says the release is similar to the disposal of wastewater at other plants around the world.