Wellbrock, the world champion, blitzed the field over the 10-kilometre open water course at Tokyo's Odaiba Marine Park to win gold by a record margin of more than 25 seconds on Thursday.

He also won the Olympic 1500m bronze in the pool on Sunday.

Paltrinieri won bronze in the open water event to go with his silver medal in the 800m in the pool a week earlier.

The pair were split by Hungarian silver medallist Kristof Rasovszky, with Australia's Kai Edwards in 12th place in his Olympic debut.

Wellbrock and Paltrinieri joined Tunisia's Oussama Mellouli as the only athletes to win a medal in both the pool and open water swimming at a single Olympic Games. Mellouli won bronze in the 1500m and gold in the 10km in 2012.

Crossing the line after one hour 48 minutes and 33.7 seconds Wellbrock's winning margin of 25.3 seconds was by far the largest in the history the event, which made its debut in Beijing in 2008.

In three previous men's marathon races, the largest margin of victory was 3.4 seconds by Mellouli, with a photo finish deciding the medallists in Rio.

Wellbrock led at all but one of the checkpoints as the race was contested in bath-like conditions with the water temperature about 29 degrees.

The stifling conditions appeared to get to Frenchman David Aubry, who dropped out of the race with about three kilometres remaining and was carried off the deck on a stretcher.

But Wellbrock said the first seven kilometres of the swim was "really easy".

"The temperature today was the biggest competitor. I beat it and I beat everything in this race," Wellbrock said.

Paltrinieri said he had battled an infection in the lead up to Tokyo so was delighted with his medal haul.

"It's been a tough two months for me and I'm just happy to be here and to be on the podium again," said the 26-year-old, who also finished fourth in the 1500m.

"It means a lot because this is my first Olympics in open water, and I've been putting myself in a position mentally to try and focus on the open water more and more in these last two, three, four years.

"That was the goal, to come here and get on the podium."

Australian Associated Press