The British Fifth Army was a field army of the British Expeditionary Force during the First World War. The Fifth Army was created on 30 October 1916 by renaming the British Reserve Army of General Sir Hubert Gough and as such it fought the Battle of the Ancre which became the final British effort in the Battle of the Somme.

In 1917 the Fifth Army was involved in the Battle of Arras and then the Third Battle of Ypres.

In 1918 the Fifth Army took over a stretch of front-line previous occupied by the French south of the River Somme and on 21 March bore the brunt of the opening phase of the German Spring Offensive, known as Operation Michael. The failure of the Fifth Army to withstand the German advance led to Gough's dismissal and the disbanding of the broken army.

The Fifth Army was reformed some months later under the command of General William Birdwood but saw little action in the remainder of the war.