| Unit Name: | Third Army |
| Dates: | 1915–1919 |
| Country: |  United Kingdom |
| Type: | Field army |
| Battles: | Battle of the Somme Battle of Arras Battle of Cambrai Battle of Passchendaele Battle of Amiens Hundred Days Offensive |
| Notable Commanders: | Edmund Allenby Sir Julian Byng |
The Third Army was a field army of the British Army during World War I that saw active service on the Western Front throughout the war.
The Third Army was part of the British Army during World War I and was formed in France on 13 July 1915, under the command of Lieutenant-General Charles Monro.[1] During August 1915 the Third Army took over trench line south of the French Tenth Army, which had to keep in position for the forthcoming autumn offensive. This made the Third Army geographically separate from the other British armies for the time being. This remained the case until March 1916, when the Tenth Army was moved to take part in the Battle of Verdun because of French losses and the Fourth Army was formed in preparation for the Battle of the Somme.
The battles it took part in on the Western Front included:
Third Army Order of Battle, August 1918.
The army was not reraised during the Second World War. However, due to various Allied deception efforts, German intelligence over-estimated the number of Allied forces based within the UK by the start of 1944. While there was no specific deception effort to create the Third Army, German intelligence believed that one had been formed from Northern Command.