In a letter released Thursday, the former Beatle urged three members of all-female band Pussy Riot to "stay strong" and called on authorities to allow them freedom of speech.
McCartney said that "I and many others like me who believe in free speech will do everything in our power to support you and the idea of artistic freedom."
The band members have been in jail for more than five months because of an anti-Putin prank in Moscow's main cathedral, and face a maximum seven years in jail.
Celebrities including Madonna and Bjork have called for them to be freed.
A judge is due to rule on their case Friday.