Two men entered the McColls shop in High Street, Belbroughton, at around 6.40am and threatened the staff.
The two men – who had their faces covered and wearing gloves – then stole a quantity of cash, cigarettes and scratch cards before leaving.
There were no reported injuries despite one of the men reportedly holding an axe.
Detective Constable Sonya Gibbs, from West Mercia Police, said: "The offenders are believed to be two men wearing gloves and with their faces covered.
"One of the men had a stocky build and was wearing quite a distinctive dark jacket with reflective stripes on the sleeves with white trainers that had a very noticeable luminous green heel.
"Thankfully, nobody was hurt but this was an understandably frightening incident and officers are supporting those involved.
"We have launched a thorough investigation to identify those responsible and a number of enquiries are currently ongoing.
"We are appealing for any information from the public, including any dashcam footage from local motorists who may have driven by at the time of the incident.
"Any information, no matter how small, may help us to identify the offenders."
Anyone with information which could help officers with their investigation, is asked to call West Mercia Police on 101, quoting incident 0061 of September 30.
Alternatively, witnesses can call Crimestoppers anonymously on 0800 555111 or by visiting www.crimestoppers-uk.org.