A man jumped out at a woman as she walked along a road in Wellingborough, knocking her over.
Police say the assault happened between 3.30am and 4am on Saturday (June 2) in Gleneagles Drive.
The victim screamed and the man, who was wearing a grey or white hooded top or jumper, ran off.
Witnesses, or anyone with information, are asked to contact Northamptonshire Police on 101.
Alternatively, they can call Crimestoppers anonymously on 0800 555111.