Yingluck, who has lived abroad since 2017 to avoid jail over a subsidy scheme that caused billions of dollars in state losses, was cleared of favouring certain companies that received government contracts, in a unanimous decision by the top court.
The ruling comes two weeks after brother Thaksin Shinawatra, the billionaire figurehead of the populist political juggernaut Pheu Thai, was freed from hospital detention on parole, six months into a commuted jail sentence for abuse of power and conflicts of interest.
Handling a complaint filed by Thailand's anti-graft commission under a military-backed government, the court in a statement said Yingluck and other defendants had followed regulations in awarding 250 million baht ($6.71 million) of government contracts. The commission has 30 days to appeal.
Like Thaksin, Yingluck fled into exile to avoid prison after her government was ousted in a 2014 coup, a key milestone during two decades of political conflict between the Shinawatra family and its allies, and a network of conservatives, elite families and generals whose interests were challenged by its rise.
Thaksin had spent 15 years abroad after fleeing in the wake of his 2006 overthrow but made a dramatic return in August to face justice. He was transferred to hospital on his first night in jail and soon after, his eight-year term was commuted to one year by the king.
His return and early release has fuelled persistent rumours that the tycoon made a behind-the-scenes deal with his powerful enemies, claims his allies and rivals have denied.
The clearing of Yingluck in the last remaining case against her could add to media speculation that she too will seek to return to Thailand. Prime Minister Srettha Thavisin, a close ally of the Shinawatras, has said the issue has not been raised.
Coverage of Thaksin's release has been dominated by expectations he will seek to exert influence on Srettha's government, or through his daughter Paetongtarn, who is Pheu Thai Party leader and eligible to become prime minister.
Srettha has repeatedly been asked by media if he would remain in charge with Thaksin now freed, questions he has rebuffed, insisting that he is still calling the shots in government. Thaksin has insisted he is retired and has been suffering from various health problems.
An opinion poll released on Sunday by the National Institute of Development Administration (NIDA) showed 43% of the 1,310 respondents considered Thaksin Thailand's most influential political figure, compared to Srettha at 22%.
