DAMS racer Valsecchi made a perfect start from third on the reversed-grid to dominate from lights-to-flag. He won by 0.4 seconds over Racing Engineering's Fabio Leimer.
Jolyon Palmer completed the podium for iSport, holding off team-mate Marcus Ericsson at Rettifilo by bouncing across the sleeping policemen.
Ericsson tumbled to seventh on the last lap after a fantastic side-by-side scrap with poleman Stefano Coletti through the Ascari chicane. Johnny Cecotto Jr and Max Chilton both passed the Swede too on the exit of the chicane, finishing fifth and sixth respectively.
Valsecchi extends his championship lead over Luiz Razia, who could only finish 16th after starting 25th due to a five-place grid penalty after his collision with Leimer on Saturday.
Lotus duo James Calado and Esteban Gutierrez kissed goodbye to their title hopes at Monza. Calado was 14th, while Gutierrez crashed out at the Parabolica on lap three, having collided with Cecotto and Luca Filippi on the opening lap.
Saturday's winner Filippi had to settle for fastest lap, after being forced to pit due to wing and tyre damage.