Thaksin expressed disappointment with the verdict and vowed to fight back against the court order.
"I haven't received justice and I will not give up," he said in a statement, pledging to continue a non-violent struggle for democracy and justice.
"I will seek justice in every way and every opportunity possible."
Supreme court ruling
The nine judges in Thailand's supreme court said on Friday that Thaksin, who was forced from power by a coup in 2006, had used his position as premier to the benefit of his Shin Corp telecoms company.
The court said that Thaksin had concealed shares in Shin Corp and geared several government telecom policies to favour the company.
Thailand froze $2.3bn of Thaksin's assets after he was forced from power and convicted of graft in absentia.
It was not immediately clear if the remaining assets, which the court found had been accumulated by Thaksin before he became prime minister, would be returned to him.
But Thaksin's supporters, nicknamed the Red Shirts, vowed on Saturday to hold a mass street rally in Bangkok in mid-March to protest against the court ruling.
They say they are seeking to force the government of Abhisit Vejjajiva, the current prime minister and a Thaksin opponent, to call new elections.
"What Thai people feel at the moment is that justice in this society is fading away," Chavalit Yongchaiyudh, the chairman of the pro-Thaksin Pheua Thai party, said.
"We have been waiting 78 years for real democracy. We are still talking about dictatorship. We want power to belong to the people."
However, Al Jazeera's Wayne Hay, reporting from Bangkok, said that there was no course for Thaksin or others in his family to appeal against the verdict.
"This is the final court in Thailand, the supreme court," he said.
"There was talk that he would go to an international court, but it has been found that he has no right to do that because he does not have the support of the state of Thailand."
The political crisis triggered by Thaksin's ouster has left Thailand and its economy in a state of unrest and uncertainty over the past four years, with frequent protests, airport blockades and three changes in government in 15 months.