Ben Delo, a British billionaire who reportedly recently moved back to the UK from Hong Kong, said he had given money to ensure a "fair fight" at the polls.
The sum offers a boost to Nigel Farage's party after a summer dominated by scrutiny of his and Reform's finances, including an ongoing criminal investigation into allegations it breached rules on political donations.
"I am honoured and humbled that Ben Delo has shown such confidence in Reform," Farage said.
"Ben knows that we are the only party that can turn the country around and reverse Britain's decline."
Mr Delo, a co-founder of cryptocurrency platform BitMEX, is now Reform's largest donor having previously been second behind billionaire Christopher Harborne, who gave £9 million ($A17 million) to the party last August.
Writing for the Telegraph newspaper, he said: "Unlike the people who express mock concern for the health of our democracy while doing everything they can to keep it as a cartel, I want a fair fight and a level playing field.
"My conclusion from Reform's recent troubles is that they should spend less time trying to raise funds to compete on equal terms and more time preparing for government."
MPs last week backed measures including a cap on overseas donations at £100,000 ($A188,661) and a suspension of funding through cryptocurrency, but the draft legislation still needs to be passed by the House of Lords to become law.
Ministers have argued the changes will prevent foreign money influencing elections, but Reform has said it is a bid to choke its funding.
The party has faced ongoing questions about its finances which overshadowed its annual conference last week following an undercover investigation broadcast on Channel 4 News.
Detectives from the Metropolitan Police are investigating after the report showed senior Reform figures appearing to discuss how to disguise donations from foreign sources.
Reform UK denies any wrongdoing and has said it will co-operate fully with the probe.
Mr Farage's long-time aide Dan Jukes, who also denies wrongdoing, and head of policy James Orr have been suspended pending an internal investigation into the allegations.
The party leader also faces an investigation by Parliament's standards watchdog over a £5 million gift he received from Mr Harborne.
Mr Farage has said he has done nothing wrong and the gift was not political.
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