The legendary Beatles musician was rumoured to have rehearsed with Dave Grohl and Krist Novoselic before the Concert for Sandy Relief event, which was streamed online on Wednesday night (December 12), along with live broadcasts on US television and radio.
Along with past Nirvana member Pat Smear, the supergroup performed a new track titled 'Cut Me Some Slack'.
Introducing the song, McCartney told the crowd: "Recently some guys asked me to go and jam with them. So I showed up... and in the middle of it, these guys kept saying, 'We haven't played together for years, you know?'
"The penny finally dropped, I finally understood that I was in the middle of a Nirvana reunion."
The track will be available to purchase as part of a live compilation album of songs from the evening.
Bruce Springsteen, The Rolling Stones and The Who were also among the long list of artists who performed at the event in New York last night (December 12).