Highly toxic Cesium-134 – the “fingerprint” of Fukushima – was found in Tillamook Bay and Gold Beach, Oregon.
The terrifying discovery was reported by researchers at the Woods Hole Oceanographic Institution.
The 2011 Fukushima nuclear meltdown was sparked by a massive tsunami off the coast of Japan caused by a 9.0-magnitude earthquake.
Since then, the entire area has been a dangerous no-go zone.
As giant waves crashed into the plant, huge quantities of contaminated water were leaked.
Cesium-124 has also been detected in Canadian salmon for the first time, reported chemical oceanographer Jay Cullen, who is leading the Fukushima InFORM team at the University off Victoria.
But researchers were quick to point out that the risk to human health is low because of the relatively small quantities of radiative material.
However, radiation levels have not yet peaked as a toxic plume makes its way towards the United States.
The Statesman Journal reported Mr Cullen saying: “It appears that the plume has spread throughout this vast area from Alaska to California.
“As the contamination plume progresses towards our coast we expect levels closer to shore to increase over the coming year.”