The tax debt amounts to around the equivalent of five billion kronor, according to several Thai media outlets.
The story goes back to 2006, when Thaksin was prime minister, and he made a criticized multi-billion profit on selling his telecom business without paying taxes.
Later that year, he was forced from office in a military coup and went into exile.
The Supreme Court is now reversing a lower court's decision and ordering 76-year-old Thaksin to pay the tax that the country's tax authorities determined in 2017.
Thaksin is serving a one-year prison sentence, convicted of corruption and abuse of power, among other things.
The Shinawatra family has challenged the country's conservative and military elites for decades, while the legal and political setbacks have piled on top of each other.
In August this year, Thaksin's daughter Paetongtarn was removed as prime minister by a court decision.
