It is believed the offender was carrying a kitchen knife during the raid at 3.45pm on Tuesday, December 6 last year.
A South Yorkshire Police spokesman said that it is reported that an unknown man wearing a woolly hat and dark clothing threatened staff inside the Post Office at Unit 5 of Tinsley shopping centre.
The man is thought to have taken cash and the cash tray from the till before fleeing the scene.
Examination of CCTV has shown that the man had been in the Post Office shop about ten minutes before the robbery was reported. It is thought that he left the shop and came back because there were customers present initially.
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Enquiries have also revealed that the man was carrying what is thought to be a kitchen knife, police said.
DC Lamb, the investigating officer, said: “Thankfully nobody was hurt as a result of this incident but this individual was carrying a dangerous weapon and it’s important we identify him.
“Since this incident was reported to police, we’ve pursued every line of enquiry available to us including examining the scene for any forensic opportunities, carrying out house-to-house enquiries, extensive area searches and circulating this footage across the force to see if any officers recognised the person featured.
“For legal reasons, we have been unable to release the footage until now but we’re hoping that someone will recognise this man and pass on information.”
Anyone with information is asked to call 101 or email [email protected] quoting incident number 697 of December 6 last year.
People can also give information to Crimestoppers anonymously on 0800 555 111.