In August 1808, Trelliard commanded a cavalry camp in Pau, France. That year transferred to Spain to fight in the Peninsular War. At first he served in the north of Spain on the Bay of Biscay where he led the 4th Dragoon Division. He became a Baron of the Empire on 9 March 1810. During the Siege of Ciudad Rodrigo in 1810, he commanded a covering force consisting four regiments of dragoons. Though he was not listed in the official Army of Portugal order of battle, which placed Louis Pierre, Count Montbrun in charge of the reserve cavalry, staff officer Jean-Jacques Germain Pelet-Clozeau stated that he led a cavalry division during the Siege of Almeida under Marshal André Masséna's personal command. After the Battle of Bussaco, Masséna ordered Montbrun to take command of the army's advance guard while Trelliard assumed direction of the reserve cavalry. In November 1810, he was assigned a task force that included two regiments of dragoons and some infantry. He conducted reconnaissances on at least two occasions during the winter. In February 1811, Trelliard led a cavalry division that was separate from Montbrun's. As the French withdrew from Portugal, he assisted Marshal Michel Ney's rear guard with three dragoon regiments on 10 March, the day before the Battle of Pombal.