The suspected stalker, a 34-year-old whose name wasn’t released, had a ticket to Swift’s concert at Gelsenkirchen’s Veltins-Arena on Wednesday, police said in a statement on Thursday.
They said that, because an initial investigation couldn’t entirely rule out a risk, he was detained during entry checks to the event.
Police said the man had made threats against Swift and her partner on social media.
They said he had been detained following tips from organisers of the event.
The American superstar has shows in Gelsenkirchen on Wednesday, Thursday and Friday as part of her Eras Tour.
A local court ordered the suspected stalker kept in detention until Saturday, police said.
They added that there was no danger at any point to Swift or to the audience. Wednesday evening’s concert, attended by about 60,000 people, went peacefully.
Ahead of the concerts, Gelsenkirchen put up signs temporarily renaming itself “Swiftkirchen” and honoured the singer on a “Walk of Fame” that commemorates local celebrities.
After Gelsenkirchen, Swift plans concerts in two other German cities, Hamburg and Munich.