Brazil has been experiencing an extreme heatwave, with the BBC reporting a heat index (which combines temperature with humidity) of 59.3C (138.7F) on Friday followed by 59.7C (139.5F) on Saturday.
Organisers Time4Fun (T4F) said in a statement that paramedics attended to Ana Clara Benevides Achado, 23, at the concert venue before she was taken to hospital. She died an hour later.
According to Brazilian newspaper Fohla de S Paulo, the fan suffered from a cardiac arrest as concert-goers waited in sweltering conditions at Estádio Olímpico Nilton Santos.
In a statement on her Instagram stories, Swift said: “I can’t believe I’m writing these words but it is with a shattered heart that I say we lost a fan earlier tonight before my show. I can’t even tell you how devastated I am by this.
“There’s very little information I have, other than the fact that she was so incredibly beautiful and far too young.”
Tonight’s (Monday’s) Eras Tour concert in Rio de Janeiro will still go ahead, before Swift moves onto three shows in São Paulo.
According to a report from the Guardian, organisers have been asked to provide new water distribution points and ambulances ahead of tonight’s concert, with entry moved forward by an hour.