The well-taken double took the tally of the former Barcelona playmaker to three goals.
The Brazilian wizard opened the scoring 10 minutes after the interval when he converted a penalty kick after Sampdoria defender Stefano Lucchini handled the ball in the area.
Lucchini was sent off after receiving a second yellow for the handball, leaving the visitors with an uphill to climb.
On 68 minutes, the once-misfiring Ronaldinho doubled the lead with a well-threaded low shot, and cheered the San Siro spectators with his famous victory dance.
Italian veteran Filippo Inzaghi put the icing on the cake with a third strike seconds before injury time to ensure his team's superiority.
The victory put Carlo Ancelotti's side in fifth place of the league table with 13 points.
Elsewhere, Lazio lost their position as Serie A leaders their after a 3-1 defeat at Bologna FC.
Sergio Volpi opened the scoring as early as the fifth minute before Italian striker Marco Di Vaio scored twice to give Bologna a commanding lead.
Tommaso Rocchi scored a consolation six minutes after the restart.
Watch the first goal
Watch the second goal
Watch the third goal