A man was hit in the face after stopping to check if his attacker was ok.
The assault took place sometime between 8pm and 8.25pm on Saturday (March 9) in Judith Road at the junction with Elizabeth Road, Kettering.
A police spokesman said: “A 33-year-old man was walking his dog and carrying a bag of shopping.
“He saw a man bent over and asked if he was ok.
“The victim was then hit in the face by the man, causing him to drop his shopping.
“The victim ran off and returned shortly after to find his bag of shopping had been taken.”
The offender was a white man, of average build and wore a hooded top, jeans and a scarf.
Officers would particularly like to speak to dog walkers nearby who may have witnessed the incident.
Anyone with information about the assault can call Northamptonshire Police on 101 or information can be given anonymously to Crimestoppers on 0800 555111.