The man wore the terrifying distinctive Halloween-style mask with only his eyes visible when he raided the SR Supermarket in South Street, Armadale at around 7.30pm on Wednesday night.
When he entered the shop he brandished a knife and demanded the member of staff open the till.
The man fled the shop with the stolen cash before police were alerted.
He is described as being white, between 19 and 25-years-old and 5ft 9inches – 6ft tall. He was wearing a dark coloured jacket with the hood up, dark jeans and dark boots.
A police spokesperson said: “Although not injured, this has been a terrible experience for the member of staff involved.
“We are appealing for anyone who may have been in the area at the time, and may have seen something suspicious, to contact police.
“We are also keen to hear from anyone who may recognise the description of the suspect.”