The club has been without a shirt sponsor for three years, but will now have Betano as its sponsor for at least two years.
The deal is expected to generate around R$16m (US$3.15m) for the football club; Betano will make its debut as sponsor during Wednesday's game against Bragantino in Maracana.
As part of the of the agreement, the club's former deal with bookmaker Betmotion will be cancelled, with the company ending the partnership that was contracted until March 2022.
Fluminense President, Mario Bittencourt, said: “We have always affirmed that the club’s appreciation would involve the arrival of a partner who understands the club’s greatness.
“This shows that, as I have repeated, we are achieving results through a management philosophy that prioritises organization, environmental stability and the recovery of our finances. We are on the right path, seeking to expand our brand and we will have a master who understood this project”.
The deal with Fluminense would be the second Brazilian sponsorship for Betano, as the company is already the main sponsor of the club Atletico Mineiro, which plays in the same league as Fluminense.
Betano has also sponsored football clubs across the globe, reaching commercial agreements with Braga and Maritimo in Portugal, as well as clubs in Romania.