McNamara, 45, of Hawaii, was filmed on Monday surfing what appears to be a 100-foot wave off Nazaré, Portugal, the same location where, in 2011, he broke the record for surfing the biggest wave ever ridden.
"I was going so fast, I could barely stay on the board," McNamara told ABC News. "I just work on staying present in the moment, and then there's no fear."
"What was going through my head was, 'This is ridiculous,'" he said.
Footage of McNamara, possibly breaking his own world record, was shot by surfing photographer Tó Mané.
The wave will have to be verified by Guinness World Records officials. McNamara's 2011 wave was initially thought to be 90-feet but was later verified through photos and videos to be 77.7559 feet high.
That height narrowly eclipsed the old "big wave" surfing record of 77 feet.
McNamara, however, is not worried about the details.
"I just want someone else to break the record so they can have fun," he said, also calling Nazaré, the site of his two biggest rides, the "8th wonder of the world."
In an interview with ABC News after his 2011 wave, McNamara said a lot of careful planning goes into riding big waves. "I set up ahead of time ... I run through it in my mind over and over. I check through all the safety precautions," he said.
"It's a very thought out, well-planned mission but once you get in the water and you're at the spot there's always the unknown and you just have to deal with it as it comes," he said, noting that once he's in the water his "100 percent instinct" takes over.
"Half the people think I'm out of my mind ... and half are just - I think they all think I'm crazy, actually."