Anis Omar Zen, 19, from Harrow, was found by police in Masefield Avenue in Stanmore after being called at 4.50pm on Monday to reports of a stabbing.
Despite the efforts of officers and paramedics he was pronounced dead at the scene a short time later.
His family has been informed and are being supported by specialist officers.
Police said a 19-year-old man was arrested near the scene on suspicion of murder. He has since been bailed pending further enquiries.
Detectives from the Specialist Crime Command are leading the investigation, supported by officers from the North West BCU.
A post-mortem examination is due to be held on Thursday.
Detective Chief Inspector Laurence Smith, said: “I am urging anyone with information to come forward and contact us. This murder happened in broad daylight, and I’m certain that it would have been wit-nessed by people who are yet to speak with officers
“We are carrying out a review of CCTV footage but I would also ask that if you have a video doorbell, or were driving in the area with a dash cam, please check the footage.
“A crime scene will remain in place over the coming days and we are grateful for the co-operation of residents.
“The motive for this murder remains unclear at this time. We are determined to find the person responsible but we need your help.”
Any witnesses or anyone with any information is asked to call police on 101 or contact via Twitter @MetCC, quoting reference CAD 5493/04Sep.
To give information anonymously contact Crimestoppers on 0800 555 111 or online at crimestoppers-uk.org.