Plant operator TEPCO believes most of the water flowed into the Pacific Ocean. It says the contaminated water leaked from a treatment pipe.
The water contained radioactive strontium which tends to accumulate in bones and can cause leukaemia.
It is the second time in two weeks there has been a leak of contaminated water from the nuclear plant, prompting yet another apology from TEPCO.
About 120 tonnes of radioactive water leaked at the plant’s water decontamination system last month and about 80 litres seeped into the ocean, according to TEPCO.
The water, once it has been used to cool the reactors, contains massive amounts of radioactive substances and is put into the water-processing facility so it can be recycled for use as a coolant.
“Our officials confirmed that cooling water leaked at a joint in the pipes,” a TEPCO spokesman said, adding that “it is possible that part of the water may have flowed outside the facility and poured into the ocean”.
The leak has since been plugged, the spokesman added, saying the utility was probing the cause of the accident and how much, if any, water flowed into the Pacific.
The plant, about 220 kilometres north-east of Tokyo was crippled by meltdowns and explosions caused by Japan’s massive earthquake and tsunami in March last year.
Radiation was scattered over a large area and made its way into the sea, air and food chain in the weeks and months after the disaster.
Tens of thousands of people were evacuated from their homes around the plant and swathes of the zone remain badly polluted.
The clean-up is proceeding slowly, amid warnings that some towns could be uninhabitable for three decades.