A man was knocked unconscious after he went to the aid of a woman who was being attacked in Ripley.
He saw her being assaulted by three men on New Street at 3.30am on Sunday.
The man went over to help her and the group surrounded him and attacked him.
He was knocked unconscious and taken to the Royal Derby Hospital.
The men were all described as aged between 18 and 20. One was described as of a stocky build, 6ft tall with short brown, spiky hair. He was wearing a plain grey t-shirt and light blue skinny jeans.
The second man was 6ft 2in tall with short blonde hair. He was wearing a white t-shirt, grey jeans and hi-top trainers.
The third man was 5ft 7in tall with dark blond spiky hair and was wearing a grey, long sleeved top and blue jeans.
Officers would like the woman who was assaulted to contact them.
Witnesses or anyone with information should call police on 101 or Crimestoppers on 0800 555 111.