Olivier Giroud's thumping header secured AC Milan's first Champions League win of the season as the Italian side beat Paris St-Germain in Group F.
The French forward netted the winner from Theo Hernandez's cross five minutes into the second half.
Milan Skriniar had opened the scoring for PSG, heading in at the back post from a Marquinhos flick on.
Rafael Leao swiftly equalised with an overhead kick from close range after Giroud's initial shot was parried.
Their victory means last season's semi-finalists Milan will remain in contention to reach the knockout stages when they host Dortmund on 28 November.
Former goalkeeper Gianluigi Donnarumma was roundly booed by the home fans, who also threw fake money towards him, as he returned to the San Siro for the first time following his acrimonious move to PSG two years ago.
Milan were much improved from their previous Champions League performances this season, with Donnarumma forced into fine saves from Hernandez and Fikayo Tomori free-kicks.
However, the evening ended on a sour note for the hosts, with Christian Pulisic withdrawn after injuring his hamstring in the closing stages.