The 34-year-old Brazilian terminated his contract with Olympiakos last week, only five months into a one-year deal. Marcelo played nine matches for the Greek club and scored three times.
"It is time to come back," he posted on his social media channels.
Fluminense published a montage of the club´s crest adorned with the player's shaggy hairstyle.
Marcelo left Real Madrid last year as the club's most decorated player. In 15 years, he won 25 trophies, including five Champions Leagues and six La Ligas.
Marcelo, who is expected to play as a midfielder at Fluminense, was part of Brazil's squad at the 2014 and 2018 World Cups.