They performed Horan's track 'Slow Hands'.
It was the first of two London tour dates at Wembley Stadium, the second taking place tonight (23 June). Find stage times here.
Watch fans’ clips of her and Niall’s performance below.
After the show, Taylor posted a pic of her and Niall soundchecking, thanking him for surprising the audience.
It’s not known whether he’ll also play tonight.
The singer paid tribute to the ‘incredible resilience’ of victims of the Manchester bombing, during her first UK ‘Reputation’ tour date, as she played the city’s Etihad Stadium on 8 June.
Some Swift fans were left furious at the Manchester Etihad Stadium dates after losing their front row seats due to ‘stadium problems’.
Meanwhile, last year, Niall Horan revealed plans to work on album two over the course of 2018.
It follows the release of his debut solo album, ‘Flicker‘ in October 2017.