A man was knocked unconscious as he walked down Spring Gardens in Buxton with his wife.
The offender grabbed hold of the 40-year-old victim’s jacket and pushed him to the ground at around 4.25pm on Monday.
The man’s wife pushed the attacker away and a passer-by stopped to challenge him.
The victim suffered grazes and a lump to the back of his head.
He was taken to Stepping Hill Hospital for treatment.
The attacker, who is described as white, of large build and around 6ft tall, ran off towards the Slopes. He was wearing a dark top with white writing on it.
Anyone with information should call PC Leigh Gyte on 101, quoting crime reference number 35251/15, or Crimestoppers anonymously on 0800 555 111.