The singer, who brought her Reputation Stadium tour to London on Friday, introduced the veteran entertainer to a packed audience for a duet of "Angels."
Williams appeared to rapturous applause and cheers, and was appropriately dressed for the occasion, sporting a Taylor Swift T-shirt.
Fans shared their excitement and disbelief on Twitter.
“Fair to say when Robbie Williams hits the Taylor Swift stage to sing Angels it's an uplifting moment. #liftedtheroof,” said Twitter user Beejayville.
Taylor Swift just brought out Robbie Williams to perform Angels. Night made 😭❤️ pic.twitter.com/t8dlsDu94Y
— that elle (@ellesmx) June 23, 2018
Rafaella-Leo wrote: “Robbie Williams just appeared on stage with Taylor Swift and I cried.”
Meanwhile another fan tweeted: “Robbie Williams singing angels with Taylor Swift is something I did not know I needed to see until I saw it”.
Videos and images of the special moment flooded the social networking site.
It follows a special appearance from One Direction star Niall Horan on Friday.
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