A woman approached another woman and asked for money before spitting in her face.
The 35-year-old victim was approached by another woman with a young child in Northampton Road, Queensway, Wellingborough, who asked for money.
When she politely refused the woman spat at her and pulled a bag from her.
The offender dropped the bag when she realised it contained nothing of value.
She is described as olive-skinned, 5ft 4in with a slim build and aged in her early 20s with dark hair scraped back into a bun. She was wearing dark trousers and a blue top. The child with her was about two years old.
The incident happened at about 8pm on Sunday April 20.
Witnesses to this incident, or anyone with information, should call Northamptonshire Police on 101 - alternatively, information can be given anonymously to Crimestoppers on 0800 555 111.