Security van staff loading a cash machine at the Santander bank in High Street, Shoreham, were threatened by a man with a handgun shortly before 11pm yesterday (Tuesday, February 27), police said.
Staff were forced to hand over a box containing several thousands of pounds, according to police.
No one was hurt in the incident, police confirmed.
Police described the robber as a white man, 5ft 10ins, in his 30s, with brown medium length hair.
He was wearing a blue hooded jacket and carrying a black pistol, said police.
After grabbing the cash box he ran away on foot in a southerly direction, according to police.
Enquiries are continuing to identify and locate the robber, added police.
Anyone who saw or heard anything suspicious in Shoreham High Street late on Tuesday evening, or who may have other information that may be of assistance to police, is asked to contact Sussex Police online or phone 101, quoting Operation Graffiti.