The boy identified as Che Labastide-Wellington, from Harrow, was at a party in Belvedere Way, Kenton, on Saturday evening.
Police said there had been a fight with a group of youths who tried to gain entry,
Che was stabbed in a later scuffle in an alleyway at about 00:30 GMT.
He was pronounced dead about an hour later.
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A 16-year-old boy was also stabbed and taken to a north London hospital for treatment. His remains in a stable condition, said police.
They said Che had been invited to the party, which appeared to be "a well-run event with appropriate adults in attendance".
They believed he was not involved in an early altercation with a group of youths denied entry to the party.
But he appeared to have been targeted later, a spokesman said.
Det Chief Insp Chris Jones said he wanted to speak to anyone who was near the alleyway between Belvedere Way and Tylers Gate when Che was stabbed.
A man aged 21 has been arrested on suspicion of murder and taken to a west London police station for questioning.
Two boys aged 15 and 16 were also arrested on suspicion of murder and bailed until mid-December.
A third, aged 14, is being held on suspicion of possession of an offensive weapon.
A 17-year-old boy who was fatally stabbed in north-west London was at a 16th birthday party targeted by gatecrashers, police say.