The man was detained following tips from organizers of the event, police said in a statement. They said he had previously made threats against the singer, German news agency dpa reported. They gave no more details of the man or the background to the incident.
Police said that Wednesday evening's concert, attended by about 60,000 people, went peacefully.
The American superstar has shows in Gelsenkirchen on Wednesday, Thursday and Friday as part of her Eras Tour.
Ahead of the concerts, the city put up signs temporarily renaming itself "Swiftkirchen" and honored the singer on a "Walk of Fame" that commemorates local celebrities.
After Gelsenkirchen, Swift plans concerts in two other German cities, Hamburg and Munich.