The fish would have picked up the pollution while swimming in Japanese waters, before then moving to the far side of the ocean.
Scientists stress that the fish are still perfectly safe to eat.
However, the case does illustrate how migratory species can carry pollution over vast distances, they say.
"It's a lesson to us in how interconnected eco-regions can be, even when they may be separated by thousands of miles," Nicholas Fisher, a professor of marine sciences at Stony Brook University, New York, told BBC News.
Fisher and colleagues report their study in the journal Proceedings of the National Academy of Sciences.
They examined the muscle tissues of 15 Bluefin tuna (Thunnus orientalis) taken from waters off San Diego in August 2011, just a few months after the accident at the Fukushima Daiichi nuclear plant.
These were animals whose parents would have spawned in Japanese waters and spent one to two years locally before heading to feeding grounds in the eastern Pacific.
Professor Nicholas Fisher was 'stunned' to find the radioactive signal in bluefin tuna.
All the fish examined in the study showed elevated levels of radioactive caesium - the isotopes 134 and 137.
Caesium-137 is present in seawater anyway as a result of the fallout from atomic weapons testing, but the short, two-year half-life of caesium-134 means the contamination can be tied directly to Fukushima. There is no other explanation for the isotope's presence.
The measured concentrations were about 10 times the total caesium radioactivity seen in tuna specimens taken from before the accident.
As a control, the team also examined Yellowfin tuna, which are largely residential in the eastern Pacific.
These animals showed no difference in their pre- or post-Fukushima concentrations.
The research is likely to get attention because Bluefin tuna is an iconic species and a highly valuable fishery - thousands of tonnes are landed annually.
But consumers should have no health concerns about eating California-caught tuna from last year, the team says