Having been linked with Angolan club Kabuscorp and Antalyaspor of Turkey, plus the MLS, this year, since it became clear his time at Queretaro in Liga MX was coming to an end, Ronaldinho has instead chosen to go home to Brazil.
Fluminense announced the deal on Saturday, with Ronaldinho hoping to help the Rio de Janeiro-based club win their third Campeonato Brasileiro title in six years.
"It was one of the things that motivated me to return to Brazil, going after a title that still do not have, and also because it is a club that has great players," the attacking midfielder said in a statement.
"It's a very cool project and I am very happy to join these great players."
Fluminense won the Brazilian Serie A in 2010 and 2012, and they sit second in the 2015 standings after 12 rounds - two points adrift of leaders Atletico Mineiro - one of Ronaldinho's former clubs.
Ronaldinho also previously played for Fluminense's arch rivals Flamengo.
In one season with Queretaro, Ronaldinho scored eight goals in 25 games, while in his last full season in Brazil - with Mineiro, the veteran struck seven times in 14 matches.