The former banker used private bookmakers registered on the Caribbean island of Curacao for the bet at odds of 37/20 — meaning he could get a $15 million payout, sources told the Sun.
The mystery gambler confidently went all-in after consulting with "Trump camp insiders," according to the report.
"Word of this bet has done the rounds and we think it's the biggest ever made on politics," one betting industry source told the UK paper.
While illegal in the US, betting on politics is popular in the UK and many other countries — and the 2020 election is shaping up to be the biggest betting event of all time.
Trump beat the odds in 2016, and gamblers appear keen on a repeat performance this time.
"Donald Trump will likely wake up with a spring in his step as more and more people think he could stay for another term," the gambling comparison site OddsChecker said Tuesday.
In the last four hours, 71% of money wagered on the 2020 US election was on Trump, the site said — which the site's head of marketing, Sam Eaton, called a "remarkable" swing.