Southern Swedish police confirmed late on Monday that the teenager, who was found severely injured at the gym in the town of Ängelholm on Saturday evening, had died from his injuries on Monday afternoon.
Police found the boy after being called out to an assault at the Friskis & Svettis training centre at around 6pm on Saturday. They arrested a 15-year-old suspect at his home immediately thereafter.
As The Local reported on Sunday, the Friskis & Svettis in Ängelholm wrote on its website that the attack involved a knife and that fellow gym-goers intervened and helped to save the victim's life.
The gym thanked its members and said it would increase its security as a result of the incident.
The 15-year-old remains a suspect and is facing allegations of murder or manslaughter, said police.