The 21st Division was an infantry division of the British Army during World War I, raised in September 1914 by men volunteering for Lord Kitchener's New Armies. The division moved to France in September 1915 and served on the Western Front for the duration of the First World War. The divisional insignia was the "triple-seven".

The New Army, often referred to as Kitchener's Army or, disparagingly, as Kitchener's Mob, was an (initially) all-volunteer portion of the British Army formed in the United Kingdom from 1914 onwards following the outbreak of hostilities in the First World War in late July 1914. It originated on the recommendation of Herbert Kitchener, then the Secretary of State for War to obtain 500,000 volunteers for the Army. Kitchener's original intention was that these men would be formed into units that would be ready to be put into action in mid-1916, but circumstances dictated the use of these troops before then. The first use in a major action of Kitchener's Army units came at the Battle of Loos.