The robber targeted the Shell filling station on Handsworth Road and stole a quantity of cash after threatening a member of staff.
His face was covered with a scarf and sunglasses during the robbery which happened at around 3.55am on Wednesday June 21.
The suspect is described as white, in his late 20s or early 30s and was wearing a grey woollen hat and gloves, a black jacket and black trousers.
A South Yorkshire Police spokesman said: "No one was injured; however the member of staff was understandably left extremely shaken following the incident.
"The offender left the area, heading towards Ashbourne Road, with an amount of cash taken from the garage.
"While we appreciate this incident happened in the early hours of the morning, police are asking for anyone who saw or heard anything suspicious around that time to contact them."
Anyone with information that could assist with enquiries, please call 101 quoting incident number 117 of 21 June 2017. You can also call Crimestoppers anonymously on 0800 555111.