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."
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.