At the start of the 1809 campaign, Grenier commanded an infantry division in Eugène de Beauharnais' Army of Italy. He fought at the Battle of Sacile on 16 April. At the Battle of Piave River on 8 May, he led a 16,800-strong corps of two divisions. At the Piave, his successful attack on the right flank caused the Austrian withdrawal. He intercepted and crushed Franz Jellacic's Austrian division in the Battle of Sankt Michael on 25 May, where he demonstrated his superior generalship. He also led his corps at the Battle of Raab on 14 June and at the Battle of Wagram on 5–6 July. At the latter battle he commanded three divisions and was rewarded with the Grand Cross of the Légion d'honneur.