A massive earthquake and tsunami caused nuclear meltdowns at the Fukushima Daiichi plant in 2011.
The company, Tepco, has been pumping in water to the plant to cool down the reactor's fuel rods.
This has meant that the plant has produced contaminated water daily, which has needed to be treated and stored in huge tanks.
The water contains extremely high levels of radioactive substances.
To combat this, Japan has decided to dilute it with seawater before putting it into the ocean.
Dozens of protesters took to the streets of Toyko on Friday to condemn the release.
Japanese fishing groups have relayed fears that the release will damage the reputation of their seafood.
China, which is the biggest buyer of seafood from Japan, has said it will block all imports following the decision despite Japanese officials and scientists saying the release is safe.
Join our commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies