District judge John Law dismissed the cases against the 21-year-old Swedish campaigner and the four other activists on the second day of their trial at Westminster Magistrates' Court.
He ruled that police deployed in the British capital in October at an environmental protest had attempted to impose "unlawful" conditions before officers arrested dozens of demonstrators.
Thunberg, a global figure in the fight against climate change, was among dozens held for disrupting access to the Energy Intelligence Forum, a major oil and gas conference attended by companies at a luxury hotel.
She had pleaded not guilty in November to breaching a public order law, alongside two protesters from the Fossil Free London (FFL) campaign group and two Greenpeace activists.
In his ruling, Law said the conditions imposed on the demonstrators were "so unclear that it is unlawful", which meant "anyone failing to comply were actually committing no offence".
Thunberg and the other defendants had faced a maximum fine of GBP2,500 ($3,177) if convicted.
Her lawyer, Raj Chada, said the case against them had been "rightly dismissed", arguing that the police stipulations "disproportionately interfered with our client's rights to free speech".
He added: "The government should stop prosecuting peaceful protesters and instead find ways to tackle the climate crisis."
- 'Ridiculous' -
Christofer Kebbon, one of the other defendants from FFL, told reporters that the five "shouldn't be here in court".
He condemned "the climate criminals who are continuing their business as usual and destroying this planet".
Thunberg, who came to worldwide attention as a 15-year-old by staging school strikes in her native Sweden, regularly takes part in climate change-related demonstrations.
She was fined in October for blocking the port of Malmo in Sweden, a few months after police forcibly removed her during a demonstration against the use of coal in Germany.
She also joined a march last weekend in southern England to protest against the expansion of Farnborough airport, which is mainly used by private jets.
Demonstrators had greeted the October forum participants with cries of "shame on you!".
Some carried placards reading "Stop Rosebank", a reference to a controversial new North Sea oil field the British government authorised in September.
Police said officers had arrested Thunberg for failing to adhere to an order not to block the street where the rally was taking place.
Greenpeace UK campaigner Maja Darlington hailed Friday's verdict as "a victory for the right to protest".
She added: "It is ridiculous that more and more climate activists are finding themselves in court for peacefully exercising their right to protest, while fossil fuel giants like Shell are allowed to reap billions in profits from selling climate-wrecking fossil fuels."
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