The former Barcelona star moved to Mexican side Queretaro in September but rescinded his contract last month and has been a free agent ever since.
He had appeared close to joining big-spending Turkish side Antalyaspor, who have already signed Samuel Eto'o and hope to bring in Victor Valdes from Manchester United, but has now elected to move back to Brazil.
"We reached an agreement with Ronaldinho," he had said. "I think within two days it'll be made official."
But Fluminense, fierce rivals of Flamengo, one of Ronaldinho's former clubs, will be the fourth Brazilian side Ronaldinho has represented during his illustrious career.
After making his name in Europe, the forward moved back to South America with Flamengo in 2011, before moving on to Atletico Mineiro.
He won the Copa Libertadores in his two-year spell in Minas Gerais, adding to the two Primera Division titles and the Champions League crown he won with Barcelona, and the 2010-11 Serie A title with AC Milan.