Lew Smart, 31, was sunbathing at Senne Cove in Cornwall when he spotted a fin in the water. Rather than stay safely on dry land, Smart scrambled to get into the water with his underwater camera - and took some amazing footage of the beast.
The 15ft basking shark is shown gliding through the water with its huge mouth agape - plenty wide enough to have swallowed the man without issue.
Basking sharks pose no threat to humans, only feeding on plankton as they swim through the ocean.
Lew has swum with sharks before, and stayed in the water for 20 minutes to observe this particular breed's behaviour.
'We get basking sharks quite frequently just off the beach in Sennen' he said.
'After I saw it about 30 metres out I decided to jump in the sea as I thought it would be quite cool to have a swim with it.'
And the 31-year-old admits this definitely isn't the scariest experience he's had with the fish, after being chased by both a tiger shark and a reef shark on separate occasions.
From May to October basking sharks come up from the deep ocean to feed on Plankton blooms. They are often spotted in British waters.