Detectives want to speak to the man, who stopped his car and helped a 24-year-old who had been attacked as he walked along North Bridge Road, Doncaster, between 1.30am and 3.15am on Sunday, August 13.
South Yorkshire Police said the attack victim was ambushed by a group of men, who left him with injuries to his head and face.
The man who stopped to help then drove him home.
PC Rob Scarrow said: "A man in a car is said to have stopped and helped the victim, before driving him home.
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“At this time, we are trying to trace the man and I’d like to appeal directly to him to please come forward.
"Both the investigative team and the victim would like to thank him for his assistance. We’re also keen to speak to him about what happened, as we continue trying to trace those involved.
“Similarly, if you know who this Good Samaritan is, or saw the initial incident, please get in touch."
Call South Yorkshire Police on 101 or Crimestoppers on 0800 555111.