South Wales Police (SWP) officers responded to a report that a 16-year-old boy was suffering a medical episode at Barry Island Pleasure Park just before 5pm on Friday.
The teenager, who has since been named as Taha Soomro, from Grangetown in Cardiff, was pronounced dead at the scene.
A 15-year-old boy from Grangetown has been arrested on suspicion of assault and is being held in custody, police said on Saturday.
Taha’s family has been informed, and they are being kept updated by officers, police said.
They urged people to avoid speculating on social media about what happened.
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Police have urged anyone who was in the park around the time of the incident and has information to contact them, especially if they have any relevant mobile phone footage.
