During the War of the Second Coalition, Grenier led a division at the battles of Verona, Magnano, and Cassano d'Adda in the spring of 1799. His command consisted of three battalions each of the 17th, 24th, and 106th Line Infantry Demi-Brigades, one battalion each of the 1st Polish Legion and 2nd Swiss Legion, 450 cavalry, and a foot artillery company. Later he fought in the Army of the Alps under Championnet. While leading 8,000 men of his division, he defeated Friedrich Heinrich von Gottesheim and 5,000 Austrians at Fossano on 16 September 1799. For a loss of 200 killed and wounded, his troops inflicted 300 killed and wounded on their opponents and captured 700 more. At Centallo on 31 October, Grenier's 7,000 troops fought 15,000 Austrians under Michael von Melas. This time he was drubbed, losing 1,000 casualties and four guns, while the Austrians only lost 200 men.