The Reserve Army was a field army of the British Army and part of the British Expeditionary Force during the First World War. On 1 April 1916, Lieutenant-General Sir Hubert Gough was moved from the command of I Corps and took over the Reserve Corps, which in June before the Battle of the Somme, was expanded and renamed Reserve Army. The army fought on the northern flank of the Fourth Army during the battle and became the Fifth Army on 30 October.

The British Army during the First World War fought the largest and most costly war in its long history. Unlike the French and German Armies, the British Army was made up exclusively of volunteers—as opposed to conscripts—at the beginning of the conflict. Furthermore, the British Army was considerably smaller than its French and German counterparts.