At the Battle of Novi on 15 August 1799, Grandjean commanded a brigade in Emmanuel Grouchy's division of Catherine-Dominique de Pérignon's Left Wing. Under his command were the 26th, 39th and 92nd Line Infantry Demi-brigades. Though holding high ground, Grouchy's division on the left flank was unprepared for the Austrian attack at dawn and was pushed back. However, the nearby French reserves were sent into the fight and forced the Austrians off the heights. At about this time the French army commander Barthélémy Catherine Joubert was killed. The French left wing repulsed a second Austrian assault at mid-morning. The French lines finally collapsed in the late afternoon and enemy cavalry cut down and captured both Grouchy and Pérignon.