Inter retained their Serie A title with three games to spare after second-placed Milan put in a weak performance to lose 2-1 at Udinese last night.
Inter's triumph, in coach Jose Mourinho's first season in charge, is their fourth straight scudetto and their 17th overall.
The champions, who will celebrate their success in tonight's home match with Siena hold a seven-point lead with Milan only having two games left.
The defeat of their city rivals gave Inter president Massimo Moratti an extra reason to party on his 64th birthday.
Fans quickly congregated in Milan's Piazza Duomo, to wave flags and celebrate the title while television pictures showed Inter players dancing in their team hotel.
Milan captain Paolo Maldini, who retires at the end of the season, had a night to forget on his 900th appearance for the Rossoneri.
The 40-year-old pulled down Antonio Floro Flores on 31 minutes to allow Gaetano D'Agostino to fire home a penalty and then failed to clear a corner before Cristian Zapata slammed the ball in at the far post early in the second half.
Massimo Ambrosini headed home in stoppage time for Milan, who are not yet assured of an automatic Champions League spot and have question marks hanging over the future of coach Carlo Ancelotti - continually linked in the media with Chelsea.
Inter have now joined Milan on 17 Italian titles with Juventus leading the way on 27. The first championship in Inter's run of four was given to them in a court room after Juve were stripped of the 2006 title in Italy's match-fixing scandal.
Inter's second scudetto was a stroll given Juve were in Serie B and Milan had points deducted from the affair while last term Juve were rebuilding in their first season back in the top flight, meaning Inter again had less competition.
Coach Roberto Mancini was sacked at the end of the campaign after falling out with Moratti but Portuguese Mourinho, who won two Premier League titles with Chelsea, has continued where the Italian left off.
Their style of play was not always pretty but their sheer physical power and the goals of Zlatan Ibrahimovic meant they have lost just three league matches this term.