On 28 January 1808, Ross purchased a lieutenant-colonelcy in the 91st Regiment of Foot, but exchanged back into the 52nd on 18 February 1808. He went to the Peninsular War with the 2/52nd, which he commanded at Vimiero, where he was mentioned in despatches. He continued to command the 2nd Battalion through Sir John Moore's campaign that winter, and five companies of the regiment in the Walcheren Campaign. Ross subsequently returned to the Peninsula to command the 1/52nd, with whom he fought at Pombal, Redinha, Casal Novo, Foz d'Arunce, Sabugal, and Fuentes de Oñoro, during Masséna's retreat from Portugal in the spring of 1811.