The Argentine midfielder will go back to his former club in January after helping Chinese outfit Guangzhou Evergrande to three domestic titles and the AFC Champions League
Conca went on loan to the Brazilian side from River Plate in 2008 before moving permanently in 2009 and spending two more years in Rio.
After switching to the Chinese Super League champions, he helped Evergrande to three back-to-back titles and an AFC Champions League trophy.
The 30-year-old has signed a pre-contract agreement with Fluminense and will be tasked with improving the club's fortunes once he becomes eligible to play in 2014, with Dorival Junior's men currently 16th in Serie A.
"Thank you, president; thank you to everyone who took part in this negotiation," Conca told the club's official website.
"Also I must thank the fans for all this affection that has been demonstrated over the years. I was very happy at Fluminense.
"I hope to go back and do things for the good of Fluminense and give back something to the fans."
Conca has been in fine form for Evergrande recently, scoring 14 times in the 2013 CSL season.