The San Siro was almost half full to welcome the 28-year-old, who wore a Milan kit as he waved to the crowd from the pitch.
- Ronaldinho from hero to zero
Milan coach Carlo Ancelotti has said the former Barcelona man will probably play alongside Kaka in a Christmas tree formation with a main striker just ahead of them.
It is not just the Rossoneri fans and their coach drooling over the idea.
"I will be exciting to play with Kaka. He is among the best in the world. He is also a friend, in the national team we play well together," said Ronaldinho, who unlike Kaka is being allowed by the club to play at next month's Olympics.
"Every player dreams of playing for Milan. It is a special emotion and I hope to bring happiness to all the fans."
The playmaker's last couple of seasons at Barca were dogged by form and fitness problems, with critics accusing him of putting partying ahead of football.
"It was a beautiful period, we won important trophies but after recent disappointments we decided together to go our separate ways," he told a news conference.
"I have always lived a regular life and I will continue like that. I am in good physical condition."
Ronaldinho's arrival at Milan has led to a huge surge in season ticket sales.
On Wednesday, when Ronaldinho landed in Italy, Rossoneri fans snapped up 5,230 season tickets out of a total of 18,518 sold during the whole of the close-season subscription period.
Supporters were unable to buy season tickets on Thursday though after a technical problem.
The club, who finished a disappointing fifth in Serie A last season, did not say if the glitch was because of the high demand.