Billy Joel's got a motorcycle collection almost as impressive as his discography ... but he's saying so long to his bikes, TMZ has learned.

The Piano Man is putting his famed motorcycle collection up for auction at Gooding Christie's inaugural action at Rétromobile New York in November.

The auction's set to feature over 70 lots from the iconic artist's expansive collection ... including a 1978 Triumph T140 Bonneville, a 1982 Ducati Mike Hailwood 900 and a 1975 Ducati 750 Sport.

Gooding Christie’s President David Gooding raved about the collab in a statement, saying ... "We are truly honored to present the Billy Joel Motorcycle Collection, and with what better location than New York City itself, a place so strongly associated with Mr. Joel and his celebrated legacy as one of the most beloved musicians of all time."

He continued ... "Billy Joel is one of the most recognized motorcycle enthusiasts in the world and we are thrilled to be presenting his prized collection at Rétromobile NY in November."

Proceeds from the sale of Billy's bikes will go toward his organization, The Joel Foundation.