Gordon, best known for his hit, "If You Could Read My Mind," passed away Monday evening in Toronto, where he'd been hospitalized. His publicist, Victoria Lord, did not say why he was getting medical treatment or release a cause of death.

The folk singer and songwriter became a Canadian national treasure, but also scored big hits throughout the '70s on the U.S. Billboard charts -- like 'Read My Mind,' "Sundown" and "The Wreck of the Edmund Fitzgerald."

Lightfoot was a 4-time Grammy nominee, and he's been inducted into the Songwriters Hall of Fame, as well as the Canadian Music HOF.

He had a rep for being a fitness nut and had remained active on the road ... touring the U.S. and Canada for years before the COVID-19 pandemic forced him to take a break.

In fact, he was still touring earlier this year, until an announcement just a few weeks ago that he was canceling all remaining dates due to an unannounced "health-related" issue.

He's survived by his 6 children and his wife Kim Hasse, to whom he's been married since 2014.

Gordon was 84.