The 109th Infantry was an infantry regiment of the British Indian Army. The regiment traces its origins to 1768, when it was raised as the 5th Battalion, Bombay Sepoys. The regiment's first action was during the Mysore Campaign in the Third Anglo-Mysore War. It was next involved in the Battle of Seringapatam in the Fourth Anglo-Mysore War, and next used in the Siege of Multan during the Second Anglo-Sikh War. Its last campaign in the 19th century was the Second Afghan War. During World War I it was attached to the Aden Brigade, which was formed to protect the important naval refueling point at Aden.