The 30-year-old playmaker, yet to shine for Flamengo since his January move from AC Milan, scored with a trademark free-kick over the wall from 25 metres that gave the goalkeeper no chance.
The 72nd-minute goal came as relief for Flamengo at Engenhao as they struggled to convert their superiority into goals against surprise finalists Boavista from a small seaside town who upset Brazilian champions Fluminense in the semi-finals.
"I dreamt of being able to thank the fans for their support after they put their faith in me from the beginning," Ronaldinho told reporters.
"Today's I've started paying them back by lifting the first trophy," the former FIFA World Player of the Year said after Flamengo had done a lap of honour.
The title doubled as the start of carnival celebrations for the fun-loving Ronaldinho who has been frequenting practices at Rio's samba schools and whose presence is a certainty at the Sambodrome nights next month.
Flamengo have earned the right to meet the winners of the second phase of the Carioca, one of a string of state championships that open the Brazilian season, for the title of champions.
In the Paulista (Sao Paulo) Championship, big guns Palmeiras and Sao Paulo drew 1-1 in their derby played in a downpour at the Morumbi.
At the scheduled kick-off time, the pitch and stands were flooded and the roads around the ground looked like rivers with fans who had driven to the match stranded.
Although Palmeiras coach Luiz Felipe Scolari insisted the match should be suspended, the referee decided to postpone kickoff by an hour. He also had to halt the match for 15 minutes in the 25th because of a floodlights failure, seconds after Fernandinho had put Sao Paulo ahead.
Palmeiras equalised five minutes from time through substitute Adriano to move within a point of leaders Corinthians.
