Oscar Hamilton was killed in September 2024 at Woodgrove Shopping Centre in Melton, northwest of Melbourne.
His killer, now aged 17, had denied he was guilty of murder, claiming he only wanted to confront the other boy rather than seriously hurt him.
But a Victorian Supreme Court jury on Tuesday returned its guilty verdict after only two hours of deliberations.
The teen looked down as he learnt his fate, while Oscar's family and friends cried.
Outside court, they said they were deeply relieved at the jury's decision.
"Justice is being served," relative Caz, who declined to provide her surname, told reporters.
"While this verdict brings a sense of legal closure, it doesn't lessen the profound grief and the emptiness we carry every day.
"(Oscar's) life was tragically cut short and we will spend the rest of our lives honouring his memory."
The jury was told Oscar, 16, was seated in the centre's outdoor area with his girlfriend and friends when his killer, then 15, approached and asked to speak to him.
The younger boy wanted to confront Oscar over allegations of sexual assault, crown prosecutor Erin Ramsay told the jury.
A day earlier, the accused killer's former girlfriend, whom he had dated for a week, had disclosed claims Oscar had assaulted her.
CCTV footage played to the jury showed the pair talking for about a minute before Oscar turned around and walked back to his friends.
The killer followed, pushing one of his friends out of the way and threatening to stab them all.
The teen then called his ex-girlfriend and asked her if what she had said was true, Ms Ramsay said.
Oscar denied doing anything wrong before the younger boy grabbed him by the collar, pushed him and then stabbed him three times in torso, including the chest.
The 15-year-old ran from the scene while Oscar's friends called triple zero and people performed first aid.
Despite their efforts, Oscar died at the scene.
Defence barrister Richard Edney argued his client did not intend to kill Oscar and he had only wanted to confront the other boy over the allegations.
"This was ... a young person who lost the ascendancy in terms of the pushing and shoving and unfortunately has produced a knife in response," Mr Edney told the jury in his closing arguments.
He said the teen's remorse after the killing, where he told others he "didn't mean to do it", indicated no intention to kill or cause really serious injury.
But a person's regret did not mean they lacked intention in the heat of the moment, the prosecutor said.
The teen had brought a knife and had stabbed Oscar to vital areas including his chest, Ms Ramsay noted.
"This is not the tragic consequence of a fight between two boys," she told the jury.
The jury began deliberating at 12.45pm on Tuesday and returned their guilty verdict just before 3pm.
The teen will face a pre-sentence hearing at a later date.
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