The former Beatle has called on the Scottish to "stay together" with the rest of the United Kingdom.
"What unites us is much greater than what divides us. Let's stay together," McCartney wrote in his open letter, part of which was published in the Guardian on Saturday.
Mull of Kintyre - his song about the Scottish peninsula where he has had a home since 1966 - was one of his biggest hits, released with the band Wings. Wings featured his first wife, the late Linda McCartney and at one point, Irish guitarist Henry McCullough.
McCartney said the island also helped inspire him to write The Long and Winding Road, the Beatles classic ballad.
More than 50,000 people have repootedly put their names to the letter ahead of next month's referendum, with several hundred celebrities and public figures lending support. Mick Jagger, Bruce Forsyth, Dame Judi Dench and Professor Stephen Hawking are also among the signatories, as is England World Cup winner Sir Bobby Charlton.
Scotland votes on whether or not to remain within the UK on September 18.