A 20-year-old man from Warton has been arrested on suspicion of robbery following a raid at a shop in Freckleton, say police.
Police were called around 9.50pm on Tuesday, February 6 after a man carrying a knife took cash from the Co-Op in Lytham Road.
The Co-op did not say whether a female member of staff who was working at the time was hurt, or how much money was taken.
In a statement, a spokesman said: "The safety of our colleagues and customers is of paramount importance and the Co-op takes retail crime very seriously.
"We are implementing a number of measures to deter both criminal activity and aid convictions."
Last June, three men in balaclavas burst into the nearby Post Office, also in Lytham Road, and attacked a female member of staff, 55, who suffered minor injuries.