The British Fourth Army was a field army of the British Expeditionary Force during the First World War. The Fourth Army was formed on 5 February 1916 under the command of General Sir Henry Rawlinson to carry out the main British contribution to the Battle of the Somme.

On the first day on the Somme, eleven Fourth Army divisions attacked astride the Albert-Bapaume road. The attack was completely defeated on the northern sector so subsequent Fourth Army operations concentrated on the southern sector, handing control of the northern sector to the Reserve Army.

The plan for the Fourth Army during the 1917 Flanders offensive (that became the Third Battle of Ypres) was to mount an amphibious invasion of the Belgian coast once a breakthrough had been achieved. As the Ypres fighting became bogged down, the Fourth Army divisions were drawn off as reinforcements until the army was effectively disbanded.

The Fourth Army was reformed in early 1918 — once again under Rawlinson — following the virtual destruction and subsequent disbanding of the Fifth Army during the German offensive known as Operation Michael. The Fourth Army spearheaded the British Hundred Days offensive that began with the Battle of Amiens and ended with the Armistice in November.