The intruder threatened the worker at Singh Costcutters in Harpers Lane, demanding money and cigarettes about 8.30pm on Tuesday 7.
He then went behind the counter and stole a quantity of cash and cigarettes before making off on foot.
The shop assistant was unhurt but shaken up.
Detective constable Steve Gilmore, of Chorley CID, said: “This must have been a terrifying ordeal for the shop assistant, although thankfully they were not physically hurt.
“I would urge anyone with any information to get in touch with us.”
The offender is described as white, aged in his twenties, wearing a black hooded jacket with a neck warmer across his face, black pants and black trainers.
Contact police on 101 quoting crime number SC1708630 with information.