The 20-year-old victim had been spending time with friends in Harrow when he was chased by the group of four or five males down quiet residential streets.
As the group pursued him he ran up to a house and knocked on the door for help, police said.
However the group then ambushed him before stabbing him repeatedly as he stood on the doorstep.
Police and paramedics rushed to the scene in Eastway Crescent at about 11.30pm on Tuesday.
The victim was taken to a central London hospital where he remains.
He suffered significant blood loss but his injuries are being treated as not life-threatening, Scotland Yard said.
Detective Sergeant Amar Patel, from Harrow CID, said: "This was a vicious, unprovoked attack on a young male who had been enjoying an evening with friends when he was chased, ambushed and repeatedly stabbed by a group of males for an unknown reason.
"We have already spoken to lots of people in the area about the incident, but I would appeal to anyone who witnessed the attack and hasn’t spoken to us about it yet to come forward.
“Any detail, no matter how insignificant it may seem, will help us to identify those responsible and bring them to justice. Any information given to us will be treated in strict confidence."
The chase is thought to have started in Elliot Drive before the victim ran into Eastway Crescent.
The suspects are described as black males in their late teens wearing dark clothing. No arrests have been made.
Any witnesses or anyone with information should call Harrow CID on 101 or Tweet @MetCC.
Information can also be reported to Crimestoppers anonymously on 0800 555 111 or online at crimestoppers-uk.org