The Brazil international has long been involved in a tug-of-war with Flamengo, with reports suggesting that the 32-year-old has become disillusioned with life at the club after being insulted by Rubro-Negro fans during a match against Internacional.
Earlier on Thursday, Flamengo had quashed rumours suggesting that Ronaldinho had been released, but the former Barcelona ace decided to take legal action against the club.
Ronaldinho claims he is owed over €16 million in salaries, bonuses and image rights, and the Labour Court in Rio de Janeiro has ruled in favour of the player.
The Brazilian Football Federation (CBF) is expected to acknowledge the court`s decision before the end of the day, which will turn Ronaldinho into a free agent.
"I would like to thank once again all the support that I received at this club and I would like to say, from the bottom of my heart, that playing at Flamengo was an honour," Ronaldinho said on his Twitter page.
Flamengo vice-president Rafael de Piro confirmed that the club has been notified of the ruling and insisted that it will appeal against the decision.
Meanwhile, the Brazilian club`s president, Patricia Amorim, has expressed surprise at the player`s decision, telling Lancenet: "We have always taken care of all matters pertaining to Ronaldinho at a high level and did so internally.
"We did not expect this attitude from him. Now, the case is in the hands of our legal department, which will take all legal actions to protect Flamengo`s interests.
"This club is bigger than any person and that will always be the case."
Ronaldinho signed for Flamengo in 2011 in a deal worth €3m after spending three years at AC Milan.