Dave Grohl and Krist Novoselic joined the former Beatle before 47,000 fans at Safeco Field during Friday's concert. Guitarist Pat Smear, who played with Nirvana for a tour and an album, also joined the group on stage.
There were no Nirvana songs played though. The group covered Beatles songs and played "Cut Me Some Slack," which it debuted during a benefit concert for Hurricane Sandy relief last year.
McCartney told the adoring crowd that he was in the middle of a "Nirvana reunion."
Later in the day, Novoselic tweeted, "Thank you @PaulMcCartney for a great show and for having us play with you all."