Kavan Brissett, aged 21, was stabbed in an alleyway off Langsett Walk, Upperthorpe, on Tuesday, August 14.
He was knifed in his chest and died in hospital four days later.
Arrests have been made in connection with the incident but nobody has yet been charged.
Detectives investigating the murder believe 30-year-old Ahmed Farrah could hold vital information about the killing and a £5,000 Crimestopppers reward is available for information leading to his arrest.
Despite repeated appeals for Farrah to come forward, he remains at large.
Farrah, who is also known as Reggie and has links to Broomhall, was captured on CCTV cameras arriving at the minor injuries clinic at the Royal Hallamshire Hospital on the night Kavan was stabbed.
He was treated for facial injuries believed to have been sustained in the same incident.
Farrah returned to the hospital the following morning but has not been seen since.
Anyone who spots Farrah should call 999 immediately.
Call South Yorkshire Police on 101 to pass on any other information on where he might be.
To speak to detectives direct, call the incident room call 01709 443507. Crimestoppers can be contacted, anonymously, on 0800 555111.