Detectives from the Metropolitan Police have launched an inquiry after receiving "a number of reports" following a Channel 4 News broadcast last week, the force said.
A Reform spokesman said: "Reform UK denies any wrongdoing and will fully co-operate with the investigation.
"We won't be commenting further while a live investigation is under way."
It comes after an undercover report by Channel 4 that showed senior party figures discussing how to disguise donations from foreign sources, which overshadowed Reform's annual conference at the weekend.
Party leader Nigel Farage's long-time aide Dan Jukes, who denies wrongdoing, and head of policy James Orr have been suspended pending an internal investigation into the allegations.
The footage broadcast by Channel 4 was part of an investigation by members of undercover group Verbatim, who posed as an American donor and his son who had paid for polling allegedly commissioned by Reform and were seeking to channel money to the party.
All allegations under police investigation relate to potential offences under the Political Parties, Elections and Referendums Act 2000, Scotland Yard said.
Under electoral law, permissible donations to a UK political party must be made by UK-registered voters, or a company registered in the UK.
A Met spokesman said: "Detectives have assessed the information provided and determined that there are potential offences requiring investigation.
"The potential offences are similar in nature to matters already under investigation by the Met's Special Enquiry Team relating to donations made to the same political party.
"As a result, these matters will form part of that ongoing investigation."
The Met's Special Enquiry Team launched an investigation in February 2025 following a referral from the Electoral Commission relating to donations made to Reform ahead of the 2024 general election, as part of which two people were interviewed under caution.
No arrests have been made and no-one has been interviewed over the latest allegations.
Detectives are also seeking to obtain relevant footage relating to the Channel 4 News broadcast, the force said.
Verbatim said it will co-operate with the police investigation.
Farage, who was filmed meeting the undercover reporters for lunch in Clacton as part of the broadcast, has said he "frankly wasn't even listening" during the encounter.
"I was told there was a global family office based in London, so the money could have come from Singapore," he told the BBC on Saturday.
"The money could come from all over the world, you know. Income could have come from all over the world into a family office based in London."
He claimed to "know donation law better than anybody" and said Mr Jukes and Mr Orr "should have done basic checks" before bringing potential donors to meet him.
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