George Corones clocked 56.12 seconds at an official event in Queensland - a new benchmark for the 100-104 age category.
He eclipsed the previous mark, set in 2014, by 35 seconds. It will now be verified by the sport's governing body.
Mr Corones, who is eligible for the record because he turns 100 in April, said he was "quite delighted" and "over the world" by the result.
"It was an exemplary swim for me, well balanced... and I was ready to hit the [wall] at the end very hard with my hand," he told the BBC.
He said he had been overwhelmed by the "roaring" crowd on the Gold Coast on Wednesday. The event, in which he was the sole entrant, had been specifically staged for him to challenge the record.
"We have just witnessed history in the making!" the Australian Dolphins swim team said on its Facebook page.
'It takes a while to get going'
Mr Corones, from Brisbane, said he was a keen swimmer in his youth but only began swimming again aged 80.
"I gave it up at the beginning of the war [World War Two], and I don't think I had a swim of any description until I retired," he said.
"I started swimming again for exercise."
He said racing "undeniably" challenged him physically, but it was manageable with preparation. On average he swims laps three times a week, and also fits in gym sessions.
"At this age it takes a while to get going... you get exhausted much more easily, but if you do it sensibly, the rewards are astronomical," he said.
He attributed his success to his pacing and technique.
"My first dozen strokes were well balanced and the trick is to just carry it on progressively, putting a little bit of increasing effort in each stroke," he said.
More about: Australia
