Billy Joel's Former Bandmate, Longtime Collaborator Jon Small Dead

Jon Small -- Billy Joel's former bandmate, longtime collaborator and one-time best friend whose wife eventually married the Piano Man -- has died.

Jon died Friday, according to a Facebook announcement shared by a longtime friend, and Billy paid tribute Monday on Instagram, posting simply, "RIP Jon Small."

He was in his late 70s ... a cause of death has not been revealed.

Jon and Billy's history goes all the way back to the 1960s, when they played together in The Hassles before breaking off to form the hard-rock duo Attila, with Billy on keyboards and vocals and Jon behind the drums.

Their partnership eventually imploded after Billy fell in love with Jon's wife, Elizabeth Weber ... who then divorced Small and married Joel.

Despite what could've been a friendship-ending betrayal, the two eventually reconciled -- and Small later became a major music video and concert director, working with Billy on projects including "Live From Long Island," "Live at Yankee Stadium" and "Live at Shea Stadium."

Small also built an impressive career outside Billy's orbit ... working with huge names including Run-DMC, Aerosmith, Garth Brooks and Reba McEntire.

Their complicated past was revisited in the 2025 HBO documentary "Billy Joel: And So It Goes," where Billy acknowledged he deserved the punch in the nose Jon gave him over the affair.

Jon and Billy had one hell of a road together ... from bandmates to bitter fallout to decades-long collaborators.