Kyle Sheehan was stabbed in the leg in Jiggins Lane, Bartley Green, at about 22:15 BST on 21 September.
A man, 23, has been arrested on suspicion of murder. Three people, aged 16, 18 and 29, arrested on suspicion of wounding have been released on bail.
Kyle had been in intensive care but died on Friday night, West Midlands Police said.
Det Ch Insp Wayne Jones: "He leaves behind a family and many friends who are distraught and struggling to come to terms with what has happened.
"We know many people were in the area and would have seen what went on that night but many are still yet to come forward.
"We appeal to you to speak to us and help us find those people responsible for killing Kyle."
A 16-year-old boy who was stabbed in Birmingham has died, prompting a murder investigation.