Former Gambian President Yahya Jammeh "orchestrated the embezzlement of nearly $1bn (£762m) of public funds", a report by the Organised Crime and Corruption Reporting Project (OCCRP) claims.
Mr. Jammeh, currently in exile in Equatorial Guinea, has not responded to the allegations, the OCCRP says.
The team of investigative reporters says they reviewed thousands of documents from the course of Mr. Jammeh's 22-year rule, which ended in 2017 after little-known presidential challenger Adama Barrow defeated him at the polls.
Gambia's Justice Minister Abubacarr Tambadou had already accused Mr. Jammeh of unlawfully withdrawing $50m from state funds over a 10-year period.
But the OCCRP say the total figure is much higher.