The 58-key upright was sold to an unidentified buyer for $602,500 at Sotheby's New York on Friday.
Its pre-sale estimate was up to $1.2 million.
It was offered by a Japanese collector on the film's 70th anniversary.
The collector purchased the movie prop at a Sotheby's auction in 1988 for $154,000.
Humphrey Bogart played Rick Blaine in the Oscar-winning World War II love story, opposite Ingrid Bergman's character, Ilsa Lund.
In a famous flashback scene, Rick and Ilsa lean on the piano at a Paris bistro. Sam, played by Dooley Wilson, plays and sings.
They toast as Rick says: "Here's looking at you, kid."