Tokyo Electric Power Co. spokesperson Naoki Tsunoda said Wednesday the company's attempt to stem the leak by injecting 1,500 liters (400 gallons) of "water glass," or sodium silicate, and another agent near a seaside pit where the water was leaking appeared to have been successful.
The leak was discovered Saturday, and radiation of more than 7.5 million times the legal limit for seawater was found just off a tsunami-damaged nuclear plant.
The water stopped leaking at 5:38 a.m. (2038 GMT) Tokyo time, Tsunoda said.
Radiation has been seeping from the plant, raising concerns about the safety of seafood in the area. For the first time, Japan's government has set radiation safety standards for fish.
Workers have been pumping water into the reactors trying to prevent a full-scale meltdown.
Officials decided to pump some of the less radioactive water pooling around the plant into the sea.
Experts said the radiation dissipates quickly in the ocean, but long term effects aren't clear.
Fukushima is not a major fishing region and no fishing is allowed in the immediate vicinity of the plant, but fishermen in the prefecture fret that demand will collapse for catches elsewhere - whether or not they are contaminated.
Some government assurances of safety have done little to quell panic. In Tokyo, for instance, there were runs on bottled water after officials said radiation in tap water there was above the level considered safe for infants, though insisted it was still OK for adults.
The Associated Press contributed to this story