Negus McClean was attacked in Westminster Road, Edmonton, on Sunday as he tried to protect his younger brother, according to his family.
Police said they were looking for nine young men seen on mountain bikes chasing Negus before the attack.
Negus, described as intelligent and mature, was the fourth teenager to be stabbed to death in London this year.
The teenager, who lived in Edmonton, was stabbed several times in the chest and thigh and died later in hospital.
Det Ch Insp Steve Clayman said officers were keen to trace a light-skinned black male, believed to be under 18, and wearing a grey or black hooded top.
The victim's 25-year-old cousin, who gave his name as Noel, said: "He was dying protecting his brother. It was a selfless act. He paid with his life.
"It was just a disgusting act of violence."
This post was made using the Auto Blogging Software from WebMagnates.org This line will not appear when posts are made after activating the software to full version.