Michael Brown, who was initially located by the British police after giving a series of interviews to the Majorca Daily Bulletin, was sentenced in his absence to seven years in jail in 2008 for stealing £36m from clients including nearly £8m from Manchester United's ex-chairman Martin Edwards.
He posed as a bond dealer and claimed royal connections to steal the money, Southwark Crown Court heard.
Brown, who organised a huge fund raising charity golf tournament on the island prior to his arrest, gave some £2.4m to the Lib Dems ahead of the 2005 general election.
City of London Police said they were pleased to hear he had been detained and were establishing contact with authorities in the Caribbean country, which has no extradition treaty with Britain. Clearly, at some stage we will look for his return to the United Kingdom, so he can serve the sentence for the fraud offences for which he has been convicted in this country, a spokesman said.
The British Foreign Office said it was aware of the reports of his arrest.
Brown used investors' money to fund the money he gave to the Lib Dems' election war chest - its largest ever donation.
The Electoral Commission ruled in 2009 the party could keep the money.