A police dog bit two men while being taken for a walk by an off-duty handler in Lundwood.
An investigation has been launched into the circumstances surrounding the incident at Lund Avenue on Saturday.
One of the men, a 44-year-old, was left with a serious hand injury and a 19-year-old man was bitten on the leg.
A spokesman for the police said the incident happened at 5.15pm when the dog was being taken for a walk by its handler.
Both victims, who police said live locally, required medical treatment for their bite injuries.
The investigation is being carried out by South Yorkshire Police's Professional Standards Department, the force said.
Anyone who witnessed the incident in Lundwood should contact South Yorkshire Police on 101 quoting incident 886 of September 14.