The off-duty policeman was driving along a street in the north of the trendy southern Berlin district, when at around 3:45am he came across the British tourists ambling along the road.
Police report that the officer confronted the men verbally and a short exchange of words ensued. He then got out of his car and followed the 19- and 24-year old visitors.
He struck the older of the two in the face and stabbed him in the buttock. The young Brit needed to be treated in hospital as a result of his wounds.
Police confirmed to The Local that the two tourists have since been released from hospital and have returned to the United Kingdom.
In his defence, the 29-year-old policeman has claimed that one of the two young men attacked him with a knife. Police report that the off-duty cop had a wound on his hand which also needed to be treated in hospital.
An investigation of the officer on suspicion of carrying an illegal weapon has now been initiated.
Police are refusing to comment on whether he will continue to serve while the investigation takes place.