Shortly after 1.30am on February 3 a man entered the Esso garage, in Downshire Way, Priestwood.
He removed what was appeared to be a firearm from his pocket, before demanding cash from a member of staff. The money from the till was placed in a plastic bag and the offender then left the premises. No one was injured in the incident.
The offender is black, approximately 5ft 10ins, aged 20 and 25 years, and was wearing black skinny jeans, a black jacket with fur around its hood, white trainers and a white scarf covering his face.
Detective inspector Stuart Brangwin, from Maidenhead Force CID, said: I would like to speak to anyone who witnessed the robbery or saw anyone in the area at the time of the incident who may have been acting suspiciously.
If you have any information which could assist the investigation, please contact our 24-hour Thames Valley Police enquiry centre on 101.