Brighton Pavilion MP Caroline Lucas was arrested outside energy company Cuadrilla's exploratory oil drilling site in Balcombe, West Sussex, last August 19.
She was among a group of protesters who had linked arms outside the site, which had become a national focal point for anti-fracking protests.
During the trial at Brighton Magistrates' Court alongside four co-defendants, Ms Lucas, 53, said she "wanted to express solidarity" by protesting peacefully.
She was found not guilty of the two charges she faced - wilful obstruction of a public highway and breaching an order under Section 14 of the Public Order Act.
Outside court, she said: "We are pleased that the court upheld our right to peacefully protest against fracking, but this judgment is not a victory or cause for celebration.
"We will continue to campaign to end fracking and only celebrate when our world is on the path to a clean energy future."
Ms Lucas's co-defendants - Josef Dobraszczyk, 22, from Bristol; Ruth Jarman, 50, from Hook, Hampshire; Sheila Menon, 42, from north east London; and Ruth Potts, 39, from Totnes, Devon - were also cleared of both charges.
Ms Lucas told the trial she felt it was important to protest about fracking in a bid to prevent the UK being locked into using more fossil fuels.
She said: "I'm haunted by the idea that my children and my children's children will turn round to me and say, 'What did you do about this overwhelming threat?'
"And I want to do all I can do peacefully to address that before it's too late.''
Ms Lucas told the trial that governments were failing to urgently address the issues surrounding climate change.
And she said she felt the protest outside Cuadrilla's main entrance was a "legitimate'' and "appropriate" way of sending a message to the Government about fracking concerns.
Last summer's anti-fracking protests at Balcombe placed the controversial extraction method under forensic scrutiny.
Although Cuadrilla did not frack at the site, its drilling operation sparked concerns that it would go on to do so in the future.
Prosecutor Jonathan Edwards told the trial that a large camp formed near the site's entrance and extra officers were drafted in because of the vast number of protesters.
A notice under Section 14 of the Public Order Act was imposed, requiring demonstrators to use a designated protest area away from Cuadrilla's entrance.
Ms Lucas told the trial it felt important and "symbolic" to be based outside the main gate during the protest, which lasted about five hours from around 10.15am.
Giving evidence, she denied being told of a designated protest area by police, adding: "I couldn't see any reason why we couldn't continue our peaceful protest."
Pc Robert Staplehurst, of Sussex Police, told the court he approached Ms Lucas, said there was a Section 14 notice and asked whether there was anything he could say to move her.
Ms Lucas - who was elected as the Green Party's first MP in the 2010 general election - replied no and was arrested.
She went on: "If we had another 10 to 15 minutes we could have come to an agreement to end it and dispersed. We weren't doing anything different than we were in the previous five hours."
A short section of footage was shown to the court showing Ms Lucas circled while linking hands with fellow protesters.
In it, Pc Staplehurst is heard to say: "I require you to move. Are you going to move of your own accord?"
He went on: "Just bear in mind you will be arrested...is there anything that I can say that is going to make you move? In that case, Caroline, I'm arresting you."
In another section of footage, Ms Lucas and fellow protesters are seen singing: "We shall not, we shall not be moved."
Under cross-examination, Ms Lucas said she did not believe the protest would have had the same impact if it had taken place away from the main gate.
And she was questioned about whether the singing of "We shall not be moved" was an indication of defiance of the police.
Ms Lucas said: "It was more the fact that we were singing together rather than the words we were singing. It's a song of strength."
Fashion designer Dame Vivienne Westwood sent Ms Lucas a message of support during the trial, and said fracking should be halted until the public was made aware of the "true facts".
Hydraulic fracturing - or fracking - involves high pressure liquid being pumped deep underground to split shale rock and release oil or gas supplies.
Fears have been raised over the potential for small-scale earthquakes and water pollution, and that a drive to exploit new gas reserves will turn the focus away from efforts to develop a low-carbon economy to tackle climate change.
Cuadrilla drilled horizontally for some 1,700ft through Micrite formation, a type of limestone, at a depth of around 2,350ft below ground level.
But later its chief executive Francis Egan said the rock underneath the drill site, at Lower Stumble, was already naturally fractured, and the company had no intention of fracking there.
The cost of policing the lengthy protests at Balcombe was nearly £4 million, prompting Sussex police and crime commissioner Katy Bourne to seek financial aid from the Home Office.
In explaining how he reached his verdicts, district judge Tim Pattinson said the prosecution had failed to satisfy him that Ms Lucas had "the requisite knowledge" about the Section 14 order being in place.
He said: "In her police interview, Miss Lucas accepted that she knew of the fact of the notice but not of its contents.
"In evidence, she accepted that she had briefly scanned it, found it to be incomprehensible and then put it to one side intending to consider it carefully later. She said that it was then lost when a banner was unfurled.
"She was distracted by the arrest of her son and the obvious pain being caused to him during his arrest. She had no idea where the designated area was.
"My finding on these facts was that the prosecution have failed to satisfy me that Miss Lucas did have the requisite knowledge. I also find, on these facts, that there is insufficient proof of failure to comply.
"It follows that I find Miss Lucas not guilty of the Section 14 offence."
On the obstruction charge, the district judge said he did not hear any evidence that any "actual obstruction" of a vehicle or person was caused by the protest.
He said: "There was reference to the possibility of obstruction to emergency vehicles but no evidence of any actual obstruction.
"All the defendants said that they would have moved if it had been necessary to permit access to emergency vehicles. None of these defendants was 'locked on' to each other or to any object.
"I readily accept that all the defendants could have moved if they had chosen to do so.
"However, reminding myself of the standard of proof and the legal test for obstruction of the highway, my decision is that I do not find that there was an obstruction in law in this case.
"The obstruction was temporary in nature. No vehicle was obstructed. The road to the north was closed in any event and the defendants were in the driveway as opposed to the main carriageway.
"Therefore on these facts, I am not satisfied to the criminal standard that any of the defendants is guilty of this offence and I formally find each one not guilty."
The district judge said that there was no wrangle over the fact that all five defendants were taking part in a public assembly.
But he added that the dispute centred on whether they "knowingly" failed to comply with the conditions imposed by the police.
The district judge said: "This is not a trial about the rights and wrongs of the process of extraction of shale gas by fragmentation of rock, known as fracking, which was the subject of the defendants' protest.
"I have already ruled that issues of climate change are irrelevant to the decisions I have to make in this trial.
"Having said this, I am quite prepared to accept, having heard the evidence from all five defendants, that they are sincere and highly motivated in their commitment to the cause of reduction of carbon emissions."
Notices informing the defendants were distributed but they were handed out in a "somewhat casual manner", the district judge said.
He added that no evidence was called from any of the liaison officers who purportedly served the notices.
And the prosecution evidence did not amount to what a "strict legal service" where a person is clearly identified and the server makes sure the person acknowledges receipt, he said.
The district judge said the conditions were "lacking in clarity" and that an accompanying map was "inadequate".
He went on: "If criminal prosecutions for failure to comply are envisaged, as they must have been in this case, it is imperative that conditions are clear. These conditions are not clear.
"In short, I have concerns about the notice on the particular facts of this case that I find it to be invalid."
Environmental groups hailed the acquittals as a "victory for the right to peaceful protest".
Greenpeace UK executive director John Sauven said: "This verdict is a victory for the right to peaceful protest but more importantly for Britain's growing movement of ordinary people opposing fracking.
"Caroline Lucas had the guts to stand up for what she believes in. In the age of expense scandals and 'cab for hire' politicians, an MP ready to put her neck on the line is a refreshing sight.
"We need to massively cut our emissions and invest in clean energy if we are to stop catastrophic climate change.
"The Prime Minister's cheerleading for fracking and promising a cap on wind farms is clearly out of touch with what people are demanding."
Friends of the Earth South East campaigner Brenda Pollack said: "The court has taken the right decision - it should be fracking on trial, not those campaigning to stop it.
"The Government and fracking firms are doing everything they can to try and steamroll communities into accepting this dirty and polluting fossil fuel - but across the country people are peacefully protesting to stop it happening.
"If ever there was an issue where protest is necessary, it is climate change. Fracking is a risk we cannot afford to take - it's bad for the planet, will not lower our energy bills and will leave generations to come in a mess they can't sort out."