Rich German, from Laguna Beach in Orange County, was paddle-boarding earlier this week when he came across the “interesting creature” in a marine protected area.
Speaking to 9News.com.au, Mr German said the sunfish, also known as a mola mola, was “massive”.
“It was almost twice as big as other ones I’ve seen, we’re guesstimating it was about nine to ten feet (2.7 to 3 metres) big,” he said.
“If you look at the photo you can see that board is 14 feet (4.2 metres) long.
“It was a really big fish. It was amazing.”
Ocean sunfish are found in tropical and temperate waters around the globe.
They are the heaviest bony fish in the world and typically eat jellyfish.
The biggest sunfish on record measured 2.72 metres (8 feet 11 inches) long and was caught off Japan.
Mr German said he’s encountered sunfish before but was surprised this one was so close, about 200 metres from shore.
He explained he usually sees them a couple kilometres out.
“They look like a mutilated alien shark that has been bit in half,” he said.
“They’re so bizarre-looking.
Sharks spotted metres from the shore at beach in Queensland
“They’re extremely docile and just lay there.”