A 19-year-old Moroccan, thought to be one of six main suspects behind the attacks, was arrested in the town of Kreuzlingen after allegedly shoplifting goods worth €24 (£18) from a supermarket in Konstanz in south-west Germany and then crossing the nearby border.
A border guard had identified the man as one of the Cologne suspects after a shop detective followed him and called the police.
The attacks, committed in a crowd of mostly north African men during New Year’s Eve celebrations at Cologne’s central station, have resulted in more than 1,100 criminal complaints, including 480 for sexual assault, and had a major impact on the political debate around the refugee crisis in Europe.
The suspect is being detained in Kreuzlingen but is expected to be extradited to Germany under the European arrest warrant.
There is some speculation that the man could be the same 19-year-old who vanished shortly after a trial in Cologne in February, where he had been handed a one-week prison sentence for stealing a bag containing a camera.
A victim had identified the man as one of the perpetrators behind the sex attacks only after the trial, and by that time he had disappeared from the home where he was registered.
Earlier this month, a 33-year-old Moroccan man went on trial in Düsseldorf accused of sexual assault during the New Year’s Eve incident.
The Cologne police chief Jürgen Mathies has admitted that many of the people involved in the attacks may never be caught, as CCTV could not be be used to clearly identify sexual assault.