The armed robbery happened at Costcutter Pharmacy in Majestic Way, Aqueduct, Telford, just after 11.30am on Monday.
The man forced his way behind the service counter where he punched the shop assistant, who suffered minor injuries.
He made off with cash from the till on foot. As he did so he was approached by other shop staff before pulling out a knife and threatening staff.
The robber then made off along an alleyway into Eleanor’s Close.
Officers are carrying out enquiries and are appealing for witnesses.
Detective Sergeant Jon Taylor, from Telford CID, said: “There was a considerable police presence in the area as we responded to the incident, and I would like to reassure residents that officers will remain in the area, both to offer reassurance and to carry out enquiries into the incident.
“If anyone has any information that can help us identify the suspect we would urge them to get in contact.
"In particular any members of the public who may have been in the area between 11.15-11.45am this morning and have dash cam footage.
"It’s believed the man was seen outside the store on Majestic Way shortly before the robbery took place.”
Anyone with information should contact DS Jon Taylor on 01952 214735 or report this online under the Tell Us About section of www.westmercia.police.uk
Alternatively, information can be reported to Crimestoppers on 0800 555 111.