The Brazilian team claimed its first title in the CONMEBOL South American club competition for the first time in its history after beating Argentina's Boca Juniors 2-1 in extra time.
The defender won not only the Champions League five times in his 16 seasons with Real Madrid but also six LaLiga titles, five Spanish Super Cups, four Club World Cups, three European Super Cups and two Copa del Rey trophies.
In all, he has been on the winning side in 18 of the 21 club-level finals he has played in.
Marcelo, who started Saturday's final and was substituted at half-time, returned to his boyhood club in February after a spell with Greek side Olympiakos following his departure from the Spanish giants.
"Real Madrid will understand. It's my most important title, at club level, because it's the club that raised me," Marcelo told ESPN.
"I'm winning a very important title with my favourite club, the club that gave me all the tools to have my career, with the employees who saw me grow up. There's nothing more rewarding than that. It's priceless," he added.
Winning the Copa Libertadores as well as the Champions League means Marcelo has joined a group of 15 players including Ronaldinho, Neymar and Julian Alvarez to conquer both intercontinental club tournaments.
"I owed a debt to Fluminense. It was written (that we would win the championship)," he said.
