Sources close to Robbie tell us he died Wednesday in Los Angeles after battling prostate cancer for about a year. We're told he'd been doing ok, recently, but on Tuesday he told his wife he wasn't feeling well, and she took him to a hospital.

We're told his wife got a call from the hospital in the early hours of Wednesday informing her Robbie had passed.

Robbie's group first formed in Canada in the late '50s as The Hawks -- including the late Levon Helm and Rick Danko -- but they gained wide notoriety when they backed Bob Dylan on his 1966 tour ... and after that launched their own career as The Band.

Robbie wrote some of The Band's most iconic hits like "The Weight," and "Up On Cripple Creek."

He also had a solo career that included the 1987 hit song, "Somewhere Down the Crazy River."

The Band was the subject of Martin Scorsese's classic 1978 film, "The Last Waltz" ... documenting their final show. They were inducted into the Rock and Roll Hall of Fame in 1994.

Robbie was 80.