Police are appealing for witnesses after staff were robbed at knifepoint at the Co-op store in Rose Green Road, Bognor.
A man went into the shop at about 5.45pm on Monday (15 July), threatened staff with a knife and stole money from the till. No one was hurt.
The man then tried to stop a car by threatening the driver at knifepoint in Obsourne Way minutes later but was unsuccessful and he ran off.
A hour later a 40-year-old man was arrested on suspicion of robbery and taken to custody.
Detective Sergeant Alex Lowe said: "Uniformed officers responded quickly to the scene and were able to arrest a suspect shortly afterwards. However we would like to hear from any witnesses who saw the robbery in the Co-op or the attempt to stop the car driver in Osbourne Way."
Anyone who can help with the investigation is asked to report online or ring 101 quoting serial 1187 of 15/07.