Spurred on by his mother's battle with cancer, and with his father by his side, William Goodge crossed the finish line in Sydney just after 4pm on Monday.
It brings an end to 35 days of pounding the pavement, striding the equivalent of two and a half marathons per day.
That's four days quicker than previous record holder, Chris Turnbull, who completed the 3800km feat in 39 days in 2023.
The 31-year-old Brit crossed the finish line at Bondi Beach, making his massive run a new Guinness World Record.
Mr Goodge set out from Cottlesloe Beach in Perth on April 15, averaging more than 100km per day at about seven and a half minutes per kilometre.
"Done a run in Centennial Park," he said in a video posted to social media just moments before finishing.
"What's up!?" he shouted to the hundreds of runners behind him who turned out to run the last five and 10 kilometres with him.
"It's obviously been quite the journey ... we start before the sun comes up and finish when the sun's gone down," he previously told AAP.
"I've seen every one of your famous animals, other than sharks and crocodiles, but unfortunately, a lot of them have been dead on the side of the road."
After crossing the line he was handed a bunch of flowers which he placed at the shoreline in memory of his late mother who passed away from cancer in 2018.
"She was the most special person in my life ... she passed away seven and a half years ago to cancer," he said.
Throughout the run, Mr Goodge has raised money for the Cancer Council of Australia in honour of his mother Amanda.
Mr Goodge said he used his grief and the strength his mother showed while fighting the disease to motivate him to do something positive instead of "waving the white flag".
"She would be proud of everything I've done. She'd also be concerned," he said with a chuckle.
The punishing run has tested Mr Goodge.
"The first nine days trying to sort of acclimatise to what I was doing was a big challenge," he previously said.
"I wasn't sleeping at night and I had deep pain in my bones and things like hip flexors ... and mentally I was tossing and turning throughout the night, hallucinating a little bit, which isn't that fun."
Mr Goodge said he suffered multiple injuries as he crossed the country, including toenails falling off and one toe that's "been rotting for the past probably a week".
"Ironically, the knees have felt OK. They've been very solid," he said.
Mr Goodge, who celebrated his birthday on the Nullarbor Plain during the run, is also the fastest Brit to run across the US.
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