Strel insisted on night swimming despite deteriorating health on Thursday to finish the entire course, ignoring his doctor's advice to stop.
Borut Strel, the swimmer's son and project co-ordinator, said on Saturday that his father had "hit point zero".
"There will be a ceremony Sunday in Belem, but he finished today," said Borut.
Strel started at the source of the Amazon in Peru on February 1 and arrived near the city of Belem, about 2,440km north of Brazil's Rio de Janeiro, on Saturday.
According to his website, he broke his 2004 record on March 17 in the town of Urucurituba.
If confirmed by Guinness World Records, it will be the fourth time that Strel, 52, has broken a world swimming distance record.
|Paramedics whisked Strel |
away for medical treatment
In 2000, Strel swam the length of Europe's 3,004-km Danube river, and then broke that record two years later when he swam 3,797km down the Mississippi in the US.
In 2004, he swam 4,003km along China's Yangtze.
The Amazon is the second-longest river after the Nile in Egypt.
Kate White, a Guinness spokeswoman, said it usually takes about two months to review an application to establish a new record.
Throughout the journey, Strel endured cramps, high blood pressure, diarrhoea, chronic insomnia, larvae infections, dehydration and abrasions caused by the constant rubbing of his wet suit against his skin, the website said.
Strel said he was also lucky to have been spared encounters with Amazon's man-eating creatures.
"I think the animals have just accepted me,'' he said. "I've been swimming with them for such a long time that they must think I'm one of them now."
But he suffered serious sunburn on his face and forehead, blistered lips, scabs on his nose and cheeks, and lost about 12kg by the time he finished.
He said there were times when he was in so much pain "that I could not get out of the water on my own... Once, they had to take me to the hospital to check my heart, but everything turned out OK".
|Many gathered near Belem to watch the |
swimmer finish the 5,265-km journey
He said on Thursday that the journey became tougher as he approached Belem.
"The finish has been the toughest moment so far,'' he said, when he was still about 100km from the finish line.
"I've been swimming fewer kilometres as I get closer to the end. The ocean tides have a lot of influence on the river's currents and sometimes they are so strong that I am pushed backward."
Chats with his psychotherapist when he was not in the water also helped him cope with delirium by discussing his pains and redirecting his thoughts to other things, Strel said.
Asked about new adventures, he said: "I am not going to do the Nile. It's long but not challenging enough, it is just a small creek. The Amazon is much more mighty."
Pictures courtesy of Amazon Swim