A 25-year-old woman was assaulted as she walked up the stairs in the Grosvenor Centre car park in Northampton town centre.
The assault happened on Friday, September 6, between 4.15pm and 4.30pm, in the stairwell opposite the main entrance, next to Northampton House and near to the bus station.
The woman was walking up the stairs between levels one and two, when she was grabbed from behind and pushed to the floor.
Police are keen to speak to anyone who was in the area around the time of the assault and who may have seen anything suspicious.
Witnesses to this incident, or anyone with information, can call Northamptonshire Police on 101. Alternatively, information can be given anonymously to Crimestoppers on 0800 555 111.