The leak occurred Wednesday as workers attempted to pump an overflow of rainwater from recent typhoons into a storage tank in the B-South area of the facility, Voice of America reported.
The storage tank was overfilled and the contaminated water leaked out where the side connects with the top part of the tank with bolts, the newspaper said.
"We believe [contaminated water] flowed into the ocean," said Masayuki Ono, acting general manager of Tepco's Nuclear Power and Plant Siting Division.
The Nuclear Regulation Authority ordered Tepco to collect soil samples near where the contaminated water leaked into a ditch that leads to the ocean, the Asahi Shimbun reported.
In August, some 300 tons of radioactive water leaked from another tank at the facility, some of which was believed to reach the ocean.