The French were fast to bring up and deploy the fresh troops in front of San Giuliano, and the Austrians were slow to mount their attack. The soon-famed 9ème Légère, the 9th Light Infantry Regiment, were thrown forward to buy time screening Desaix's forces as they deployed, surprising the head of the Austrian column in the process. A French counterattack beat back the lead Austrian brigade. Zach brought forward a grenadier brigade in response and they resumed their attack. Faced with a crisis, Napoleon sent Desaix forward again and ordered a supporting cavalry charge requested by his general. French guns blasted the Austrians with grapeshot at close range. Further back, an Austrian ammunition limber exploded. In the temporary heightening of confusion, the Austrian grenadiers were charged on their left flank by French heavy cavalry and disintegrated. This precipitated a general collapse of the Austrian centre.

However, at the very moment of victory, Desaix was killed by a musket ball.