After performing in the US, Mexico and Argentina, Swift has brought her Eras tour to Brazil for concerts that were repeatedly delayed by the pandemic. She will play in Rio and São Paulo, but her show in Rio will be postponed.
Writing on Instagram, Swift said: "I can't believe I'm writing these words but it is with 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."
Swift later added that her show in Rio would be postponed due to the extreme heat.
Swift performed her first concert in Brazil on Friday evening. Local media reported that temperatures in the Brazilian stadium reached up to 60C.
She posted to her Instagram story: "I'm writing this from my dressing room in the stadium.
"The decision has been made to postpone tonight's show due to the extreme temperatures in Rio.
"The safety and well-being of my fans, fellow performers, and crew has to and always will come first."
Footage posted on social media shows Swift pausing her show to ask for water to be distributed to her fans.
In one video, she is seen saying: "There's people that need water right here, maybe 30, 35, 40 feet back.
"So whoever is in charge of giving them that, just make sure that happens. Can I get a signal that you know where they are?"