Kavan Brissett, aged 21, was knifed in an attack in broad daylight off Langsett Walk, Upperthorpe, on August 14, 2018.
He underwent emergency surgery but could not be saved and died in hospital four days later.
Detectives are treating the stabbing as a targeted attack but so far nobody has been charged.
They want to trace Ahmed Farrah, who is believed to hold vital information about the stabbing.
Despite repeated appeals for information on his whereabouts he has not yet been located.
His passport was never recovered after he disappeared.
Farrah was captured on CCTV at the minor injuries clinic at the Royal Hallamshire Hospital, where he was treated for facial injuries, on the same night that Kavan was stabbed.
He returned to the hospital the following day but then disappeared from Sheffield.
Farrah, who is also known as ‘Reggie’ and has links to Broomhall, is believed to have been injured in the same incident in which Kavan was stabbed.
The last known sighting of him was in Cardiff the month after Kavan’s death.
Friends, relatives and acquaintances have all been visited and harbouring notices have been served, warning that anyone helping him evade arrest faces prosecution.
A £5,000 reward for information leading to his arrest has also been offered by Crimestoppers.
Anyone with information should call South Yorkshire Police on 101 and quote incident number 827 of August, 14, 2018.