Negotiations had been underway since Atletico played in FIFA's Club World Cup in December.
Ronaldinho said at the time he wasn't sure whether he would remain with the club in 2014.
A World Cup winner with Brazil in 2002, Ronaldinho led Atletico Mineiro to the Copa Libertadores title last year.
The former Barcelona playmaker has said he remains hopeful of playing in this year's World Cup, although he hasn't been summoned by Brazil coach Luiz Felipe Scolari for several months.
The 33-year-old Ronaldinho will not play for Atletico Mineiro this weekend because of a suspension for a red card at the Club World Cup.