The victim was taken to hospital from Crown Street, Acton, on Friday but died from his injuries.
A 16-year-old suspect was arrested and remains in custody in South London, police said.
Officers are continuing to question a 17-year-old arrested near the scene on suspicion of murder, the Met said.
Police believe they know the identity of the dead youth but it has yet to be formally confirmed.
Det Ch Insp Rebecca Reeves said the street would have been busy at the time of the stabbing and asked for anyone with information to come forward.
She added: "We are particularly keen to hear from anyone who saw or heard an argument outside a cafe on Crown Street."

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McIlroy, 28, suffered a recurrence of a rib injury at the Players Championship.
However, he said on Tuesday there would be "no limitations" on his swing when he opens up at Erin Hills on Thursday.
"I'm totally fine to play. It's just a matter of limiting my practice," said the world number two, who confirmed he will play in the Scottish Open in July.
The rib stress fracture, which McIlroy suffered in the close season, flared up as he lost a play-off to Graeme Storm at the South Africa Open in January.
McIlroy was ruled out for a number of tournaments in January and February but after returning to action, he felt the injury again at the Players Championship.
The Northern Irishman was forced to pull out of the PGA Championship at Wentworth and also missed the Memorial Tournament in America, which meant he has had no warm-up events in advance of the season's second major.
However, the 2011 US Open champion was upbeat on Tuesday as he spoke of his preparations for the event in Wisconsin.
"I don't feel like I need a run of events to get back into it," added McIlroy, who will play in the Scottish Open at Dundonald in order to fine tune his links preparation for the following week's Open Championship.
"Preparation-wise, instead of hitting five bags of balls I'll hit two. If you hit 100 drivers a day and you swing at 120mph, it's going to take its toll a little bit. So it's just a matter of managing that."
The long-hitting McIlroy added that he wasn't upset to see the rain falling heavily on Tuesday on the already lengthy 7,693-yard course.
"I wasn't crying when I saw that rain this morning. That's a long golf course that's only going to play longer," continued the four-time major winner, who is using a new putter this week.

Police said officers from the North West counter terrorism unit searched an address on Peakdale Avenue, Crumpsall, Manchester on Friday.
The suspect, 26, was arrested the following day on suspicion of offences under the Terrorism Act.
Police would not comment on the nature of the alleged offence but said it believed it caused "no threat" to the community.
A second teenager has been arrested on suspicion of murder after a 17-year-old boy was stabbed to death in a west London street.
[NEXT_CONCEPT]
Rory McIlroy has insisted he is "totally fine" to play in this week's US Open as he returns to action after a month out because of injury.
[NEXT_CONCEPT]
A man is being held in Manchester on suspicion of terrorism offences.