The former World Cup winner and FIFA's World Player of the Year in 2004 and 2005 walked out on Flamengo and alleged the club owed him 40 million reais in unpaid wages and bonuses.
Atletico have signed the 32-year-old Brazilian until the end of the year and he could make his debut on Wednesday in a league match against Bahia.
"He is not just any player, he is Ronaldinho," said club president Alexandre Kalil. "I have a reputation as a madman but I am not mad enough to turn down the chance to sign Ronaldinho."
Atletico, one of two big clubs in Belo Horizonte along with Cruzeiro, finished 15th in the league last season and have won the title just once before in 1971.
Ronaldinho joined Flamengo in January 2011 and in his first year his goals helped them win the Carioca State championship and to finish fourth in the league, a result that got them into the Copa Libertadores, the South American equivalent of the Champions League.
However, this year they have fared poorly in the state championships and never made it past the group stages of the Libertadores.
Ronaldinho's relationship with the club began to sour when Flamengo failed to pay him on time.
Last week he said enough was enough and walked out, serving a writ for 40 million reais in unpaid salaries and bonuses.
Club officials said the debt was much lower, between four and five million reais.
Ronaldinho's career has gone rapidly downhill since 2006 and his drop in form is often blamed on his partying lifestyle.
He reached his peak during a five-year spell with Barcelona, then spent three less successful years with Milan.
Ronaldinho also peaked early at international level, helping to win the 2002 World Cup, performing poorly four years later and being left out of the squad in 2010.
At the start of this year coach Vanderlei Luxemburgo left Flamengo after a series of public disagreements with the player who has also been criticised by his successor Joel Santana.