The body of a California man swept out to sea by a surge from Japan's tsunami last month has washed up on the Oregon coast, about 500 kilometres away, US authorities say.

The body of Dustin Douglas Weber, 25, who was taking photographs of the tsunami as it reached the US when he was swept away, was found on April 2 by someone walking on the beach near Warrenton, Oregon, at the mouth of the Columbia River.

Washed away ... Dustin Douglas Weber

"The body had been in the water for several weeks," Christopher Young, Oregon deputy state medical examiner, said.

He said significant decomposition required identification using dental records and the official cause of death was listed as drowning.