A jury unanimously found the boy not guilty after being forced to decipher crude diary sketches and memes about infamous terrorists to decide his fate.
His family gasped and cried when the verdict was read out in Brisbane Supreme Court on Thursday before the teen left the building hours later flanked by security guards.
The boy - now aged 17 - had been accused of planning nail bomb attacks to promote "anti-technology" beliefs after being inspired by the American "Unabomber" Ted Kaczynski.
The teen, who cannot be identified for legal reasons, pleaded not guilty to carrying out acts in preparation for, or planning, a terrorist act.
At the start of the trial, jurors were told they would need to find the boy had committed all three elements of the charge to return a guilty verdict.
The jury had to agree the teen had carried out acts to plan or prepare for an attack, had intended to cause death or significant damage and had intended to influence people or governments to advance an ideology.
Defence barrister Laura Reece spent hours cross-examining terrorism expert Andrew Silke over whether the boy instead had "mixed, unstable and unclear extremism" rather than a particular ideology.
Professor Silke conceded the teen's writings about detonating a nail bomb amidst 20,000 people at Brisbane's Labour Day march was "not consistent" with the Unabomber's preference for targeting powerful individual people rather than mass casualty attacks.
The boy was about to turn 16 when he rode his scooter around Brisbane's suburbs in July 2024 to buy nails, metal pipes and ingredients for explosives, the jury heard.
The jury was shown dozens of text messages, online posts and hand-written diary entries that detailed the teen's fixation on infamous terrorists including Oklahoma City bomber Timothy McVeigh and the Christchurch mosque shooter.
The teen's school friend told the jury they exchanged "edgy jokes" including ones about bombing the Labour Day parade.
"This was our sense of humour," the friend said.
The friend said he exchanged "incel posts" with the teen including memes that depicted the Unabomber as "a big strong guy in high school who gets all the girls".Â
The teen used his iPhone and laptop to search for "where is Peter Dutton located" as part of an alleged plan to use bombings to oppose the Liberal Party's then-policy of building nuclear power stations in Australia.
"Who are you trying to kill?" a friend of the teen texted.
"Members of the Liberal Party," the teen replied.
The boy was also obsessed with a fictional group of wild west outlaws as depicted in the video game Red Dead Redemption 2, the jury heard.
He sketched child-like drawings in designer-brand notebooks of the cowboy gang's leader and of bombs exploding.
"I think I've gained an autistic interest in bombs but it's going to be tough to shake it off," the teen wrote in his diary.
The teen had engaged in "clearly dangerous experimentation" with household chemicals, Ms Reece told the jury on Monday during her closing statement.
But the central issue at trial was his state of mind or intention at the time of the acts, she said.
"He was a troubled kid. He was experimenting not only with explosives but with ideas and beliefs," Ms Reece said.
"He was seeking out extremist material from wildly contradictory sources from the dark corners of the internet."
The teen had written in his diary about serious mental health symptoms and was affected by his parents' separation, Ms Reece said.
The jury returned its not guilty verdict on Thursday after deliberations spanning three days.
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