The victim, a 77-year-old woman, was approached by another woman who snatched her purse from her handbag.
When she attempted to run off, a man came to the victim's aid and managed to retrieve her purse before the offender ran off.
The incident happened on Sunday in Longden Coleham at about 1.30pm.
The offender was white, in her 30s, 5ft 6ins to 5ft 8ins, of a slim build, with mousey brown shoulder length hair.
She was wearing a navy blue fleece, blue jeans and light coloured trainers.
Police say they are also eager to speak to the good Samaritan who helped the victim.
He is asked to call 101, quoting incident 306S of Sunday April 5, to assist police with their inquiries.
Anyone else with information or who saw a woman of this description in the area at the time, is also asked to call police.
Alternatively, information can be given anonymously to the independent charity, Crimestoppers, via 0800 555 111 or www.crimestoppers-uk.org