He joined the Rio de Janeiro side after terminating his contract with Mexican club Queretaro, where he spent almost a year, Fluminense said in a statement on their web site.
The 35-year old, who previously played in Brazil for Gremio, Flamengo and Atletico Mineiro, said he was coming home to try and win Brazil's Serie A, one of the few medals missing from his trophy cabinet.
"That was one of the things that most motivated me to return home to Brazil, in search of a title that I don't have, as well as this being a club with great players," he told the club's website, adding that regular appearances at Rio's Maracana stadium were also a big draw.
"I am very motivated. When I was with another club in Rio de Janeiro we only played at the Engenhao (stadium) so the possibility of playing at the Maracana is really exciting to me."
One of Brazil's most decorated players, he won the Champions League and La Liga with Barcelona and Serie A in Italy with AC Milan in a globe-trotting career that included a stint in France with Paris Saint-Germain.
Neymar (L) and Ronaldinho
Image credit: Imago
He also won the Libertadores Cup with Atletico Mineiro and the Rio de Janeiro state championship with Flamengo and was a World Cup winner with Brazil in 2002.
Fluminense, who are in third place in Serie A, said he would begin training on July 27, meaning he will miss at least their next three league matches.