Pojaman na Pombejra had been facing a three-year jail term in connection with the sale of companies by her then-husband Mr Thaksin.
But the Bangkok Appeals Court found that there was insufficient evidence to support a 2008 lower court ruling.
It is the first significant court ruling since Mr Thaksin's sister became prime minister earlier this month.
Ms Pojaman was found guilty of evading tax of 546m baht ($16.3m; £9.9m) in a 1997 shares transfer in the family telecoms business.'Close relationship'
The BBC's Guy De Launey in Bangkok says the court's dismissal of the tax evasion case may be seen as vindication for the supporters of Mr Thaksin.
They have always insisted that the charges against the former prime minister and his family were politically motivated.
Ms Pojaman is a significant figure in Thailand; she is widely thought to have played a role in forming the governing Pheu Thai party, and choosing the members of the cabinet, our correspondent says.
Despite her divorce from Mr Thaksin three years ago, many observers believe the couple still have a close relationship.
Mr Thaksin was ousted in a coup in 2006, and lives abroad to avoid a two-year jail sentence for abuse of power.
His youngest sister, Yingluck Shinawatra, came to power earlier this month but Mr Thaksin is widely seen as the governing party's de facto leader.
However, Mr Thaksin has said he will not meddle in the government of his sister but would advise her when needed.