the Fukushima No. 1 plant from a different storage tank to the one where a leak was found in August, it said.
Some 430 litres of the toxic water had leaked from one of 450-ton tanks due to the heavy rainfall brought by recent typhoons, a TEPCO official said, admitting that "contaminated water may well have flowed into the sea".
The tank holds water filtered to remove caesium but still containing strontium which accumulates in bones and can cause cancer, if consumed.
The contamination level compares with government limits of 100 becquerels per kilogramme in food and 10 becquerels per litre in drinking water. A becquerel is a unit of radioactivity.
Later on Thursday, a TEPCO official said the flow was halted after a tonne of contaminated water was transferred from the leaking tank to a new storage container, Jiji news agency reported.
According to the report, the leak began at around 8.40am on Wednesday (1140 GMT Tuesday) and was halted by 3.30pm on Thursday afternoon.
TEPCO has long struggled to control waste water at the plant ever since the earthquake and tsunami caused some of Fukushima's reactors to melt down.
The company poured thousands of tonnes of water onto runaway reactors to keep them cool, and continues to douse them.
Thousands of tonnes of radioactive water are being stored in temporary tanks at Fukushima, and the new leak was found at a section where five 450-ton tanks are placed on a slightly slanted ground, the official said.
"Workers were storing water very close to the edge due to the typhoon rainfall," he said. "As a result, the water overflowed from the edge and fell outside the gutter."
A water-level gauge was placed at the edge of only one of the five tanks, he said.
It is the second time in as many months that highly radioactive water has leaked from storage tanks surrounding the stricken plant, which was heavily damaged by a March 2011 earthquake and tsunami.
In August, some 300 tons of toxic water was discovered to have leaked from a separate tank, with part of it believed to have flowed into the Pacific Ocean.
TEPCO said it had informed the Japanese government of the latest leak and the country's Nuclear Regulatory Authority has ordered the company to stem the flow and remove any contaminated soil.