The dog, which was on a lead, had locked his jaws around the man and there was no other way to remove it, police in Düsseldorf said on Thursday.
While they could give no explanation for the dog suddenly biting its handler, they said that all the officers involved were shocked. The handler only suffered minor injuries.
The surprising case came about as officers were searching an overgrown area for a car thief who had fled from a stolen minibus at a petrol station.
Officers had followed three carjackers along the Autobahn from Düsseldorf towards Krefeld and later managed to catch two of them.