The 17-year-old, who cannot be named, stabbed 16-year-old Irfan Wahid in the chest in Leeds on 10 February.
The defendant, who was cleared of murder at Leeds Crown Court, said Irfan had punched him to the ground after seeing him chatting with the girl.
Jurors heard the girl, 16, dated the defendant in 2016 and spent time with Irfan in the days before he was killed.
Irfan was getting off a bus on Harehills Lane when he spotted the 17-year-old with her.
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The defendant told the jury he had swung a knife, which he carried in his bag to defend himself, only after being attacked and had not intended to kill Irfan.
Sentencing will take place on 15 September.
Hundreds attended Irfan's funeral in February.
Cousin Sadia Khan said: "He was very open, he was the loud one and he was just so friendly to everyone."
Another cousin, Idrees Fazil, said: "Even at school I think with his friends he was obviously a very popular kid - for his smile, his laughter and his jokes."
A teenager who stabbed a boy to death in a fight over a girl has been convicted of manslaughter.