The 3rd Light Horse Brigade was a mounted infantry brigade of the Australian Imperial Force (AIF), which served in the Middle Eastern theatre of World War I. The brigade was initially formed as a part-time militia formation in the early 1900s in Victoria. In 1914, the brigade was re-constituted as part of the AIF. The brigade first saw action while serving with the Australian and New Zealand Army Corps during the Gallipoli campaign where they were noted for their attack during the Battle of the Nek. After being withdrawn to Egypt in February 1916 they were involved in the Sinai and Palestine Campaign until the end of the war. They were attached to a number of different formations being part of the Anzac Mounted Division in March 1916 and the Australian Mounted Division in June 1917, who they remained with until the end of the war. After the war, the AIF light horse regiments were demobilised and disbanded; however, the brigade briefly existed as a part-time militia formation in New South Wales until 1921 when its regiments were reorganised into cavalry brigades.