While not revealing the water's radiation level, TEPCO said the water did not escape the complex, Kyodo News reported.
The leak came to light after a leak detector sounded an alarm about 3 a.m., local time, TEPCO said. A plant worker found the water in a building housing the desalination unit and suspended the desalination process.
The desalination unit removes salt from radioactive water after cesium and other radioactive materials are filtered out.
Swim Week Miami Beach 2014 [PHOTOS]