The teen looked down as he learnt his fate in the Victorian Supreme Court on Tuesday afternoon.
He had admitted he was guilty of manslaughter over Oscar Hamilton's death, but prosecutors rejected his plea and took the case to trial.
The jury was told Oscar, 16, was killed in September 2024 at Woodgrove Shopping Centre in Melton, northwest of Melbourne.
Oscar 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) is not a targeted attack," Mr Edney told the jury in his closing arguments.
"This was ... a young person who lost the ascendancy in terms of the pushing and shoving and unfortunately has produced a knife in response."
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.
Ms Ramsay said a person's regret did not mean they did not have the intention in the heat of the moment.
The jury began deliberating at 12.45pm on Tuesday and returned their verdict just before 3pm.
Members of Oscar's family and friends cried as the verdict was handed down.
The teen will face a pre-sentence hearing at a later date.
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