The Brazilian World Cup winner, who now plays for Queretaro, was approached by a fan holding a pen during open play in his side's 2-1 defeat by Atlas.
The referee stopped play and 34-year-old Ronaldinho begrudgingly signed a Brazil shirt the supporter was wearing.
Ronaldinho scored 33 goals in 97 appearances for Brazil in a 14-year international career.
He went on to score a consolation goal for his side in injury time from a direct free-kick.
It is Ronaldinho's first season in the Mexican league, having signed for Queretaro at the beginning of September.