It is the first time a huge migrating fish has been shown to carry radioactivity such a distance.
"We were frankly kind of startled," said Nicholas Fisher, one of the researchers reporting the findings online on Monday in the Proceedings of the National Academy of Sciences.
The levels of radioactive caesium were 10 times higher than the amount measured in tuna off the California coast in previous years. But even so, that's still far below safe-to-eat limits set by the US and Japanese governments.
Previously, smaller fish and plankton were found with elevated levels of radiation in Japanese waters after a magnitude-9 earthquake in March 2011 triggered a tsunami that badly damaged the Fukushima Daiichi reactors.
But scientists did not expect the nuclear fallout to linger in huge fish that sail the world because such fish can metabolise and shed radioactive substances.