The teen, identified in court documents as Youssaf Khaliif Nuur, had been charged with murder but the district court in the southwestern town of Gothenburg found him guilty of involuntary manslaughter and aggravated assault.
"Youssaf Nuur, through his heedless behaviour, caused Alexandra Mezher's death. He is therefore guilty of involuntary manslaughter," the court wrote.
A court-ordered examination found Nuur was mentally unstable, and this may have meant that he did not know that the stabs to Mezher's thigh would cause her to bleed to death, the court said.
Mezher, a 22-year-old of Lebanese origin, was stabbed as she tried to break up a fight in the centre for unaccompanied minors.
Nuur also attacked another person who tried to help Mezher, resulting in the conviction for aggravated assault. Nuur neither denied nor admitted the events, telling the court he had no memory of what had happened.
After undergoing psychiatric care, Nuur was ordered deported from Sweden and banned from returning before 2026.
Nuur's exact age has not been determined. He told the court he was 15, but the prosecution argued that he was at least 18. After examining bone x-rays and dental records, the court sided with the prosecution.