A statement from Sussex Police read: “Officers were called to Berwick Post Office in Station Road at around 3pm to reports of a robbery.
"The suspect had left the scene and an investigation was launched to locate him.
"A 52-year-old man from Seaford was arrested later in the evening on suspicion of armed robbery and possession of a firearm. He remains in custody at this time.”
Police said this incident took place in Berwick on October 17.
Detective Inspector Steve Shimmons said: “The investigation into this incident is ongoing, but we are not currently looking for anyone else in connection.
“There will be an increased police presence in the area into the morning, and I would like to thank the community for their patience and understanding, particularly during the initial phases of the incident.
"The main road through the village has now been reopened."
Police concluded their statement by asking anyone who saw what happened or has relevant video footage to contact them online or via 101, quoting serial 791 of 17/10.